As an avid biker I can tell you this is an awesome idea!

There are already a lot of ride-outs happening, most of them are organized through forums - this makes things difficult and not 'realtime'. An site/app like this would really hit the spot.

If you ever need feedback from a biker, I would be happy to lend a hand.

One thought - you should allow users to give their email to get an update when their city is supported (Im in London, UK).