I know that the free space on my ipod indicated by itunes is not always accurate but on my ipod it said that i had 1.2gb free and then after sync, where no additional content was added, the amount of free space fell to 600mb :s What happened to all that memory?
try a hard rest on the device(hold power button then hold again to turn on) try clearing your multitasking bar and any open web browsers. You should probablly be good after that
Memory and Hard Drive space (what the OP is actually talking about) are two totally different things. Did you update an app or something? The recent retina display update for the Oregon trail game pretty much doubled its size from 600mb to 1.2gb. When you sync your device, iTunes updates files you've downloaded.
That's really strange..do you have "automatically sync new apps" checked on in itunes? The only explanations I can think of are you recorded video on your device or downloaded something, and that took up more space. Otherwise...