easy
40D
period.
Stupid answer!
I was in the very same position as you waspy and it was comments like the above that made the whole process a million times harder/time consuming.
My best advice is go and have a little try of both the cameras. Sit down think about what it is you want from the camera and choose based on your own impressions and needs. Their both excellent cameras and im sure whichever you choose you be really happy.
I was new to DSLRs and went and had a play. I loved the idea of weather proofing on the canon, but the undeniable digital punch the D90 had in low light at high ISOs/and really easy menu system did it for me and i havent looked back.
If you need any answers about the Nikon D90 id be happy to give you them.