Copyright and Eminent Domain

Can Congress legally restrict the copyright terms if it wanted to?  Or, having granted a property right, are they restricted to claiming eminent domain, and issuing compensation for seizure of private property as required by the Constitution?  They certainly seem capable of extending copyright (having previously done so). 

Perhaps one could argue that a copyright is essentially a state-granted monopoly rather than a property right, and is thus subject to various restrictions, that Congress has the right to change and regulate at any time.  Of course, those with vested (and yet unvested, if Congress extends it again) interests in promoting the term "Intellectual Property" would no doubt disagree.  Perhaps we should start calling it "Intellectual Monopoly Rights" and see if it remains as popular.

Dunno.  Food for thought.