I've been working on adding a few resampler to gmmp for the last week that will allow playback of sample rates higher than 48khz. It also allows the proper resampling of surround sound files down to stereo (gmmp would only use the first 2 channels before). I hope to have this completed soon.
Older devices might not have cpus fast enough to handle the resampling (droid eris wasnt able to) but I am hoping most phones will be able to do it. I've tested on the galaxy nexus and htc thunderbolt and they could handle 24/192 flac files.