I've had the same problem for a long time, even after upgrading from an iPad1 to an iPad2, and even after updating iOS (currently 4.3.3). The only sure way I've found to successfully complete a full backup+sync with iTunes is to ALWAYS completely turn off the iPad FIRST (ie, hold the power button until the slider appears and power it off), then turn it back on, dock, and sync.
If you forget to do this, and iTunes hangs during the backup process, I've found that the simplest way to get out of this state is to leave iTunes alone; instead, while your iPad is still docked, hold both the iPad's power and home buttons until it restarts. iTunes will eventually notice the device went away and abort the backup. However, more often than not, the backup will now be corrupt, so it's also best to go into iTunes' Preferences, select the Devices tab, and delete the iPad's backup. Immediately sync again to get a brand-new fresh backup.
My guess is that one of the iPad apps I regularly run gets into a state that blocks the backup for some reason -- similar to how an open/running app in OS X can prevent you from ejecting a drive because some file is in use. My iPhone is also running iOS 4.3.3, but my iPhone has never had any problem backing up.
This is a bug that has existed for a long time, considering how many similar threads there are. It would be nice if Apple would read or acknowledge these reports, investigate them, and do something about it.