Here's a really great idea that's available from a US and UK suppliers cant imagine why I haven't stumbled across it before
HUB-ee wheels they are small wheel hubs that contain an integrated motor, gear sensor and motor driver details here
Makes them easy to mount and hack impressive
At $35 a wheel it's a bit spendy.
www.sparkfun.com/search/results?term=hub+ee
A little costly yes Lumpy but no space needed on the base, already have an opto to count turn, no need for a motor drive... I think for a little robot it can be interresting. More, the tyre can be remove to use track unstead. For myself I am happy winstn60 has show it to me.
Its certainly a neat design and hitting the right price point is key I guess. One thing I've noticed in the last 2-3 years is as the robotics hobby grows so does the cost.
The more great technology I see the more I want ))