What is the most hackable Roomba?
The 4000 series has a serial port that uses a special connector (it looks like an SVideo port) that is easily accessible.  On the newer ones you have to remove the cover it looks like.  Website descriptions here. 

The commands can be found here.

You'll probably want something wireless to control it remotely and it looks like Sparkfun has you covered for $100.That will present itself as a serial port via bluetooth if I read it correctly.  

I'm not sure your Roomba will be able to get the beer though, but that's just a few dozen hacks away :)