This issue is currently being investigated in this thread: Moving fields in form edit mode does not work
FileMaker Inc. is aware of the issue and have had partial success in replicating the issue but they are still trying to figure out why some Mac systems with Lion installed exhibit this problem and others do not.
Thank you for posting.
As PhilModJunk (thank you) notes, this is a particularly confounding issue. Perhaps some additional details about your systems will help us pin down the cause.
Please review your installed programs for any extensions or plugins; make a list of any you find.
Have you used any external input devices at all (drawing tablet, external mouse or trackball, USB security fob?).
Any software that re-maps key shortcuts, or captures/sends messages between programs?
Any OS-command line utilities?
Anything you can add to the list of details may help us narrow down the possible cause.
*edit I overlooked that TSTuatara had noted what I previously wrote.
Have you ran recovery on the file to see if any corruption exists?
Can you smart drag? Start to drag the object and hold the shift key. This is used to keep objects in line as you drag them.