Update: Having tested this on both Leopard and Tiger, I now agree that the Leopard issue is more serious than the Tiger issue. Under Leopard, instead of just a file in progress disappearing, the entire source directory may be lost if a move operation is interrupted -- the only fix seems to be a Terminal 'cp' of the source before the Finder error dialog is cleared. Until we have more details from Apple on the scope of the problem, do not use the Finder to move files -- copy instead.
The Mac-loving web is abuzz with reports of a problem moving files in the Leopard Finder. If you're saying to yourself, "Moving files? You mean copying files, don't you?" -- nope, actually moving files, done by holding down the Command key while dragging a folder or files from one volume to another. This trick, a lightly-documented holdover from OS 9, can come in handy if you really truly don't want to leave a copy of the files in the original location; perhaps you're intending to delete them anyway, and this is one step instead of two. The inverse trick, forcing a copy instead of a move for intra-volume file drags, is done by holding down the Option key while dragging -- note the presence or absence of the green + icon telling you whether the files will be duplicated in the target or not.
Anyway, the aforementioned bug in the Finder is this: if for whatever reason your target disk gets disconnected during a file move -- a USB or Firewire cable is yanked, power failure, or a network interruption for a remote server volume -- you're likely to have problems with your moved files. In particular, whichever file was in progress when the connection dropped may disappear from both the source and target folders, never to be seen again. This is understandably upsetting and certainly cause for alarm and fuss, except for one minor point: this isn't a new problem in 10.5. The issue with file corruption or loss during a move goes back at least to Mac OS X 10.3 Panther and quite possibly further. A recommended workaround is simply not to move files; copy them, and then go back and delete the originals if desired.
What does seem to be compounding the issue for some Leopard users is instability in the SMB networking stack. If remote NAS or fileshare volumes are prone to dropping off mid-transfer, then the issue may be presenting more often than it had in previous systems. Some readers have noted that this is particularly troublesome if you're trying to clear off a drive for backup use -- au revoir, old files, au revoir.
While we strongly suggest not using the "move files" trick for anything critical, and we'd dearly love to see this issue fixed in the Finder, we also would like to gently remind our readers that everything that goes wrong is not necessarily, automatically, decidedly Leopard-related.
Thanks to everyone who sent this in.