I have the 30D and have used the 40D quite a lot. I don't think the differences are compelling - unless you want Live View, which isn't a deal breaker for me - but I'd probably get the 40D if I was in your position and didn't mind the extra cost. The 30D is still an excellent camera though, and I doubt if you will regret the choice if you go for it.
The biggest drawback of the XXD series, for me, is the AF system. The 7D seems to be a great improvement and is the first intermediate Canon to give Nikon's D300/D300s a run for it's money. That will be my upgrade, when I can afford it, unless I decide to go back to Nikon. I shot Nikon for many years in the film days, and still have a soft spot for them. More emotional than rational I suppose, but it's just a hobby for me and there's no harm in that.