I know this a common problem, but my experience has been a little freakish. I am using an HTC Desire 2.2 and usually try to maintain approximately 40 MB space free. I have installed the bloody gigantic Google+ which took up about 20 MB and there was an option to move it the SD card, which I did. Apparently only some of the 20 MB was moved, I think about 12 MB. Okay no problem, I thought, used Google+ for some time, not my thing, so I uninstalled it and couldn't believe that I'm still short the 8MB of space!
My question is where did it go? And can I get it back? 8mb is precious on the Desire.