As always, Gripster's advice is solid...both are great cameras. The best thing to do is to go hold them and check out the controls and see which seems more comfortable to you. I'm a 30D owner myself, and it is a very nice camera. For me, the rumored features of the 40D weren't compelling enough for me to wait for it. The good news is that these cameras and lenses hold their value very well (probably even better than our Bimmers), so you can always sell and upgrade later.
'11 535i Mojave/Cinnamon Sport Auto/Sport Pack/Dynamic Handling/Premium 1&2/Head Up
Current: '07 Porsche 997S Midnight Blue
Gone: 330i SG/Terra/Poplar 6MT/Sport/Premium/Comfort Access