The Nikon D90 has the edge in dynamic range (or, latitude, if you will) but the Canon seems to work a bit better in low light situations.
That said, the D90 is one-third of the price, so Canon can go stuff itself! In other news, Panasonic just released a competitor, the GH-1, which is also a 12 megapixel camera with HD video capability.
Check this super-comprehensive comparison report:
DSLR sensor ratings