I've been using runkeeper with a mount I found on amazon.
http://www.amazon.com/gp/product/B002IIZZTS
Runkeeper is great and is free. I've been using this mount for about a month now and it's great. You can detach your phone from the mount if you need to and it covers the entire phone with a snapping "case" that really protects the phone. I also feel like the phone is safer since it's totally enclosed and not just snapped into place. Unless your whole bike falls the phone isn't going anywhere no matter how many bumps you hit. I didn't use mine on the stem but instead used an old reflector mount that fit perfectly so it's higher up on my handlebars. If you have a case on your phone you'll have to remove it to fit it in the casing though. Not a huge deal though.