There are a couple of these already.
For the iPhone:
Lunch Adventure! Welcome [website] price: free (working but features still being added)
urbanspoon [app picks] price: free
Where To? [app with more than just restaurants, might not pick for you] price: $2.99
As the developer of
Lunch Adventure! Welcome I am somewhat biased. But I enjoy using it because it works on everything that has an internet connection (iPhone, Blackberry, Computer, Mac, Windows, Linux, PDA, etc.)
I've used Urbanspoon and it is nice for the iPhone but the first version kept crashing, and it seems to think i want to drive any distance it tells me and it sorts by area not by distance from me. So it wasn't all that smart, i am preparing to develop an iPhone app, but until then i just use my Lucnh Adventure.
I've never used Where To? because it costs money.