I used to have Nikons (D1X,D100, D2H, D70) and switched to Canon (1Dmk2) for the following reasons:
- Nikon is constantly behind in technology (look at the release of the D2h with 4mp while Canon already had the 8mp 1Dmk1 of 18 months on the market...or take the 20D as an example and the D200 ghost that seems to take ages to replace the ancient D100))
- Canon has superior high ISO performance. While with ISO 400 and a Nikon I was always worrying about the noise, I shoot the Canon at 800 without any hesitation
- Lens choice and price
Cons:
Nikon D2 series have very sexy bodies
and are more intuitive to use.
But, as said by others you can't really go wrong when buying a consumer DSLR from Nikon or Canon. The 350D and D70 are both fine machines (although in the 350D you get 2mp more and better ISO performance over the D70)
I must say that I would not switch back to Nikon, having owned the Canon equipment for a year now.