We're using one of the 2" and one of the 0.75" versions of these magnets:
http://www.magnet4less.com/index.php?cPath=1_95 . These seemed to be the least expensive ones we found. With 3:2 gearing (gearhead:wheel), they work fine. When we tried 2:1, we need more traction, so we will probably be going larger or adding additional magnets.
Keep your distance at least 0.25" from the pole, as these are very strong Neo magnets. If you're touching the pole, your minibot won't be going anywhere. Also be careful handling them, since there is a large pinch hazard. On a flat surface, the 2" one can support 200 lbs.
One other item to consider - since these have an intergral steel cup, there may be some question about their legality. They are marketed as "cup magnets", so we think they are safe, but you might want to order some without the cup (not as strong) just to be safe.
Happy hangin' !