I had this problem with my PC (windows 7, 64-bit) and my iPad. After messing with it for a long time, I finally found the answer (bad USB driver). The following two step process should help resolve these kinds of problems more quickly.
The first step is to verify that the iPad (or iPhone) is really connected to your computer. This can be (really) checked using a program called USBDeview. You can get this (very small) program from http://download.cnet.com/USBDeview/3000-2094_4-10614190.html or http://www.nirsoft.net/utils/usb_devices_view.html. Don't worry, it's not Spyware or Malware.
Just run the program and sort on the 'Connected' column. If your iPad or iPhone doesn't show up as Yes (under 'Connected') then your iDevice is not properly connected to your computer. Could be a cable problem. Could be an iDevice problem. Could be a USB hub problem. Note that your iDevice might be listed as what it is (iPad or iPhone) or it might be listed as 'Apple Mobile Device USB Driver' in the 'Description' column.
Apple has a number of ideas to resolve this sort of basic connectivity issue, including switching USB ports, resetting your iDevice, rebooting your iDevice, rebooting your computer, etc. You will need to try them until your iDevice shows up in USBDeview as 'Yes' under Connected.
If you iDevice shows up as 'Yes' under connected, it should also be visible in the Device Manager (found in the Windows 7 Control Panel under Hardware and Sound). You might find it under Portable Devices or it might be under Universal Serial Bus controllers.
The iDevice may or may not show up in the Windows Explorer. On some machines it does, on others it does not. Why is not clear.
If the Idevice is properly known to Windows and iTunes can't see it, then any number of other things could be wrong. Apple has a list. See the very useful page over at http://support.apple.com/kb/TS1538. Testing shows that the Apple Mobile Device service must be running. Check this using Task Manager (started by right clicking the taskbar). Note that the iPod service and the iphlpsvc service must also be running. These services have different names under the Processes table versus the Services tab of Task Manager.
AppleMobileDeviceService.exe - Apple Mobile Device
iPodService.exe - iPod Service
ItunesHelper.exe - iphlpsvc
Step 4 of http://support.apple.com/kb/TS1538 checks if the Apple Mobile Device USB driver is installed. This is essential and a common source of problems. In at least some cases, Windows will install the 'MTP USB' driver. Indeed, Windows will reinstall this driver if you uninstall it. The MTP USB driver is essentially a Windows bug (possibly caused by Microsoft). You MUST replace the MTP USB driver with the Apple Mobile Device USB Driver. Step 4 explains how to do this. Note that you MUST use the 'Have Disk' approach. Otherwise, Windows will just reinstall the invalid MTP driver.
The Apple doc indicates that you can find the correct driver at C:\Program Files (x86)\Common Files\Apple\Mobile Device Support\Drivers. Perhaps this is correct on some systems. You may find the correct driver in C:\Program Files\Common Files\Apple\Mobile Device Support\Drivers.
iTunes could make this a lot easier by checking if the iDevice is known to Windows (what USBDevier does) and if the Apple Mobile Device USB driver is properly installed for the iDevice. Sadly it doesn't perform either check. Note that iTunes does if some of the related services (see above) are actually running.
The same problem,
If you have Windows 7 Ultimate N, it appears that the iPhone and iPad will not show up in Windows Explorer, thus you cannot retrieve photos from them. The "N" means that Windows Media Player is not installed, which prevents the iPhone and iPad from being recognized by Windows Explorer.
To solve it, you must download Windows Media Player from Microsoft and install it.
Search for: Windows 7 Ultimate N Windows Media Player
Once installed and the computer is rebooted, your iPhone and iPad will show up in Windows Explorer.
Thanks pschaeffer, Step 4 did the trick. The correct driver was in C:\Program Files\Common Files\Apple\Mobile Device Support\Drivers. Under Device Manager -Universal Serial Bus controller, Apple Mobile Device USB Driver now appears for my iPad, not just my iPhone. iTunes rcognizes my iPad now and works normally.
It took me a lot of google-ing to find this solution.
I did as pschaeffer suggested and downloaded USBDeview. Worked perfect and showed my "Apple Mobile Device USB Driver" was connected (yes), but the MTS USB Driver was not. This goes on my troubleshooting flash drive.
Windows7 Pro and iPhone 4s here. As pschaeffer suggested, I went to "Control Panel>Hardware and Sound>Devices and Printers". There both drivers wear listed. The MTS USB Driver had a troubleshooting caution triangle on it. I right clicked and the option "Troubleshoot" was listed. I clicked it and it listed the trouble and an option to "Apply this fix". I did and it replaced the MTS USB Driver with "Apple iPhone".
And that is how my problem was fixed. Thanks pschaeffer for the start and direction. Just found another way to fix the same problem.
I have tried all the suggestions here, and nothing works. When I look at USBDeview, I see the Apple Mobile device shown as connected and the MTP device shown as not connected. When I look in the devices and printers, I can see the iPhone with a yellow warning icon, but when I troubleshoot it fails to update anything. I am at my wits end on this. Any other ideas from anyone?
I've given up and just downloaded a third party tool called DiskAid which lets me copy my photos from the iPhone which is what I wanted to do all along. For some reason, even though the iPhone is listed as not connected and with a yellow ! for the MTP driver, external tools like Disk Aid can see it and copy the files that I need.
Maybe I should just go back to being fully apple based so things like this won't happen.