First, the British Museum has had the Rosetta Stone for longer than we have had the Southwestern United States, which were also taken by force. Do you insist we give them back?
Second, property rights accrue to individuals, not to collectives (although sovereignty over territory, as in the case of the Southwest, is another matter). The Rosetta Stone was dug up and its ownership transferred--legally or illegally makes no difference, as Egypt would still demand it if a French soldier had purchased it for ten francs at a curio shop. For Egypt to assert property rights over it, just because it was dug out of Egyptian mud, makes as much sense as the South African government asserting ownership over half of the world's gold, just because it was dug out of South African land. If an individual owner for the Rosetta Stone can be identified, let him come forth and file a claim.
Heck, I don't care one way or the other.
It's just a rock with a bunch of scribbling carved into it, for heaven's sake.
Besides, this thread is almost as old as the rock is. How'd it get started up again, anyway?