It depends what metric you're comparing. In raw power the Raspberry Pi in its own class - 700MHz ARMv6 processor, OpenGL ES 2.0 GPU and 256 MB RAM.
IO-wise you've got a fair few GPIO headers (
with a huge expansion board planned as well) as well as numerous other outputs (HDMI, 3.5 audio, composite, ethernet etc.).
The footprint is that of a credit card, so most Arduinos are a little smaller than this I think?
But all in all they are in totally different leagues, and as the Raspberry Pi is actually cheaper, I think it's a no-brainer.
The only downside I can see is the community. The Arduino will have a far superior community for a long time to come...