A GPS course, sponsored by AOPA or someone else, could acquire similar kudos over time.
I agree about the success of the AOPA aeros course, but I think it's probably a success because self-preservation dictates that virtually all pilots would take some training before flying aerobatics. It's a well-structured course, and there's nothing else out there.
The AOPA radio-nav course is very good for those who would like to use radio navaids to help fly themselves around, and would arguably be more sensible than an IMCR for many people who take an IMC with no real intention of using it "for real". However, few people seem to do it and I would suspect that an AOPA GPS course would suffer a similar fate.
I think any GPS "rating" would need to be more official; remember though that I'm talking about something that would allow you to use a Garmin 530 as a primary navaid (in the same way that you could use a VOR), rather than a blanket requirement for every PPL with a pocket GPS.
So what sort of format would be taught in a GPS course.
I'm thinking of a (fairly) model-independent demonstration of competence, using the simulation mode to do some basic tasks - diversions etc. - and some classic gotchas like the famous invisible N866. Probably not some of the fancy IFR features (they could be included in an instrument refresher?). Something a bit like the FRTOL practical, maybe, and at similar cost? Like the FRTOL it could be complete self-study or you could use taught courses. Any "rating" would be valid only with a sufficient avionics installation.