It would appear the 10.7.3 update fixed problems for some, but caused problems for others. Apple may have fixed it already, so try downloading the 10.7.3 Combo updater again if you haven't done that in the past 24 hours or so.
Here is the link: http://support.apple.com/kb/DL1484. I suggested doing that in step 4, in case nothing else works.
Here are some things to try first that may alleviate it, in order of increasing inconvenience:
1. Try creating a new, temporary user account. Log in under that account to see if its behavior is any different. If it is then it will provide a clue toward resolving the problem. Delete the temporary account when you are finished with it.
2. Go to System Preferences > Network. Click the lock to authenticate, if necessary. Then click the gear icon under the network interfaces like so:
Whatever service order you have set, drag Wi-Fi either to the top (assuming it's not already there) or away from the top if it was already there. Click OK and then Apply. Sleep the system then wake it again to observe any changes.
3. Click Wi-Fi in the above then click Advanced. Select all the networks your computer has ever joined and delete them with the "-" button. OK then Apply again. Select your network under the AirPort icon to re-join it. Sleep and wake again to see if that did anything.
4. If you have not done so already download and install the 10.7.3 Combo updater. It is a more comprehensive installer than the one available using Software Update, and may fix some problems.
You may have done some or all of these already but perhaps there is something you have not thought of. If nothing helps we may just have to wait a bit longer for Apple to release a fix.
All steps were completed with the exception of #4. The file I am downloading from the inserted site does not contain a valid checksum. Have attempted the download 3X and each time upon attempting to open I will receive an error window "unable to open - Invalid Checksum".
No change to the problem, I have reported, was observed after completing 1-3.
OK, last thing first, go to Apple menu > System Preferences > Network, check the box "show Wi-Fi status in menu bar" and click Apply. Now you will have the AirPort icon in the upper right of your monitor.
Next, find your 10.7.3 download and delete it. I cannot explain why its checksum is invalid but the file is corrupted and useless so get rid of it.
Look for it in the Downloads folder, which ought to be in your Dock:
... and drag that bad file to the Trash.
If your Downloads folder is not in your Dock, go to your Finder to find it
Then, click the link I supplied above and download the Combo updater again.
OK, one step forward anyway...
XWINUSR, go to the following similar thread where I just posted a lengthy response that may apply to you too:
Navigate to the post date-stamped about the same time as this one. It's on the last page or close to it.
An update to my last entry!
If I let the computer go into a sleep mode on its own (10 minutes - Default Value set in Energy Saver) it will reconnect to the network without a problem. If I force a sleep mode (wait for a full minute) it will NOT reconnect to the network on its own. Good Luck with that.
That's certainly an interesting clue XWINUSR. Apparently the system does one thing when you command it to sleep, and it does something else when Energy Saver commands it to sleep. Perhaps Apple will pick up on something there.
As a short term solution it's not bad. Just walk away from the computer, let it sleep when it's good and ready.
There have been anecdotal reports of Bluetooth being a factor, but they are about as common as anecdotal reports of just about everything else.
This problem reminds me of the rash of complaints regarding random Airtunes dropouts. All sorts of suggested fixes, most of which amounted to superstition, until the complaints eventually subsided. There have been no complaints for years now. I doubt anyone knew the ultimate cause, but it is likely a combination of factors was involved. That is probably what is happening with this one.
A similar symptom for me as well. Yesterday morning after I clicked to wake it, connected just fine; however, the rest of the day and evening it would no longer auto connect after waking from sleep. This morning after waking from all night sleep it auto connected just fine. I have already let it go to sleep and then wake it. No go. Had to go and manually select me (I am the only one in the preferred network), before it would connect. As I have shared with 2 support advisers, I never had the first issue with connecting after sleep, ever since I downloaded Lion when it first came out last year. Still no issues on subsequent updates until I downloaded 10.7.3 last week and instantly started having the problem. Installing the Client Combo over the original has not helped. One of the advisors said that engineering suggested I try disconnecting bluetooth. i told him that since I had no other mouse nor keyboard to try...that was not an option. Of course it will auto connect if I restart...so I guess like many others, I have no good option other than to try and be patient while we wait for the gurus to sort out this problem.
I only mentioned the Bluetooth factor as I have also had some problems in connecting my wireless keyboard and mouse (both Apple) in the past. Batteries on each occasion have been 100% and "discoverable" but just wouldn't connect.
I now digress to the experts to find this and fix (hopefully). Will still look at a few things upon request.