Especially now you can get the Boe Bot as an Arduino Shield http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/arduino/List/0/SortField/4/ProductID/820/Default.aspx (just add an Arduino).

The Ardunio Robot Kit won't be available until July. But I think it will be quite popular. Some alternatives: http://www.robotmesh.com/df-robot/4wd-mobile-platform-with-romeo http://www.robotmesh.com/df-robot/miniq-2wd-complete-kit-based-on-arduino