You'd probably be interested in reading this article:
http://www.nytimes.com/2003/04/13/nyregion/13LONG.html?pagewanted=2
The Downtown has open mike on Tuesday nights - each band who signs up gets, like, 10 minutes to play...there are more details in the article, but in a nutshell, if they like you, you may be asked to play for their homegrown show on a wednesday night - and possible airplay on wbab
the village pub has sunday open nights, i believe - there are a buch of places around that have similar deals.
Good luck!