An acid based wheel cleaner (one that contains phosphoric and hydroflouric acids) would be ideal. It will eat the rust right off.
Keep in mind 2 important points:
1) this nasty acid based cleaner is only for chrome plated wheels - it will damage anything else
2) The rust will come back, since it's where the plating is gone from a steel surface.
The only proper "cure" is to have the wheel replated.
I believe Pep Boys stocks the Eagle One chrome wheel cleaner - it's this nasty type.