Mike,
You are correct, it's a big jump I haven't made yet myself as the Rebel XT bodies keep cranking along with more than 6,000 clicks each! That said, the viewfinder is spectacular, the feel and especially the low light / high ISO performance is what would make me go Canon 5D versus a 30D.
Plus being an Ikelite housing devotee I can keep using my ports, cords, eTTL2 capable strobes, etc. So maybe my thinking of upgrade path isn't the same as for others. with different systems to consider replacement costs in...
As Todd mentioned thinking of shooting a Canon 5D possibly in the future I did a lot of hand wringing before letting my Canon 10-22mm EF-S lens go.....But I did, swapping it to a wedding photographer buddy for his MINT Canon 70-200mm F4 L and some $$$$$$$$. On the Rebel XT this is a 112-320mm equivalent focal length view. Plenty for my needs....
Then I a bought a NEW Sigma 15mm EX and have even used as a wide angle on the surface with great results. Underwater as Todd states, it's a no brainer....Focuses to 6", no rings to attach, etc. Being a 15mm Fisheye, when / if I move to a Canon 5D, I'll have a 180 degree Fisheye ready to shoot.
Just a thought for you, Mikey! 
dhaas
P.S. - Answering emails real quick remotely. Offline until tonight and then in all day tomorrow. Send the Flame Throwers In!!!