GPS altitude data is OK, but never great. For the curious, do some research on the ASTER Digital Elevation Model. I have quite a few altimeter / barometer apps on my phone, including one that reports ASTER altitude using GPS location lookups. It's also interesting to lock (calibrate) a barometer app to local airport readings (quite accurate), then get differentials computed for iPhone readout (Hunter Research makes one). All kinds of things to experiment with, but at the end of the day, you can get some good comparative / relative numbers for your rides, but nothing you'd use to land a plane. Not at a cyclist-friendly price, anyway.