So my post just got eaten. I will try again.
Pros of molding (using Urethan and silicone as Brandon suggested):
-Reproduceability
-Can be done in a garage once the molds/dies are made
-Doesn't tie up machining resources
-Is a fun job for freshmen
Cons:
-Lacks strength of aluminum wheels
-Less material flexibility
-Improperly mixed resin will have very poor structural properties (worst case is putty)
A quick overview of the process: two parts are mixed, a resin and a hardener (as in an epoxy). This is poured into a premade mold and allowed to harden. You then break it out of the mold and debur with a sander/file. Brandon's idea of molding silicone right onto the wheel as tread sounds pretty awesome. For mold design and process questions, hit up Brandon, me, or Mike. Mike and I worked with this stuff back in 2005 and apparently Brandon now gets paid for this
For material ideas see SmoothOn's website (The same company that makes the urethane foams ubiquitous in MORT robots.)