There is also the http://www.plaxo.com/ approach where you invite other people to update their own contact information. However some caution is probably in order, as:

- they are now owned by Comcast
- they have been flamed in the past for spamming your friends
- they haven't been playing nicely recently with people using their API to sync to the iPhone

I should say that I haven't actually used them myself, I was considering using them and http://www.nuevasync.com/ to sync contacts and calendar data to my iPhone, but got scared off when Plaxo blocked access to their API from nuevasync's servers.