The keys are interchangeable between the mac and pc version but unlike WOW, MacSoft Games did the port and owns the rights vs. Epic for the PC Version, so it would still be illegal to use it that way. Likewise, since Epic made (or atleast sponsored) the Linux port of the game, it is completely legal to use the key from the WinXP version for that installation.
Now would be as good a time as ever to pick up a copy of Mac UT2004, since its only about $30-40USD; and its porter, Ryan C. Gordon, is going to update it soon to be universal! Talk about support!