I've been playing with the NikePlus app that's native to iTunes and will let you run to a workout and play music with only one device. I'm going to be spending some significant time on a treadmill this winter and didn't want to be wearing a Garmin AND an iPod. The accelerometer sensor for your shoe is only $19.

Plus buckeyeoutdoors takes sync'd files from both Nike and SportTracks so my trainer can work with me.