Is there an easy way to get rust off of a bicycle rim?

Depends on how bad it is. Light rust will clear up with the use of the same kind of polishing compounds used on chromed car parts, if it's heavier you can use a scotch brite/brillo pad. Be aware that this can scuff the surface though. If it's really bad the rust can be sanded down, but then you need to apply some sort of alternativev surface treatment(paing/wax etc) or the rust will come straight back.