I found my site listed on google only a couple of weeks after I launched it, without even submitting it. Seems one link from an already listed site is enough in some circumstances.
Also, I gather that if you put a link to your website in a signature on a google-listed forum, and post often, that can trick Google into thinking there are lots of independent links to your site rather than just you posting it all the time! A friend reckons this is how he got his site into Google. Dunno if it'll work, but every little helps.