From lifehacker:
Get a free, anonymous, disposable phone number at Craigsnumber

To use Craigsnumber, you just need to fill out a form telling the site how long you want to use the number before it expires (minimum 1 hour, max 1 week) and the number to forward the calls to. This is potentially a very useful service, but there a couple of caveats: 1) Craigsnumber, despite the name, does not appear to be affiliated in any way with Craigslist. I'm assuming the idea behind the name is that you might want to use a Craigsnumber when posting on Craigslist. 2) Craigsnumber has no real terms of service explaining what they do with your number, so beware - nobody wants spam phone calls.

If you're looking for something a bit more permanent that still allows for advanced call screening, check out GrandCentral.

Good idea.

shaaaaaweeeeet!

craigsnumber is no longer giving out anonymous numbers, however you can still receive calls anonymously at http://letscall.me