A friend of mine has cracked an iPhone a few months ago (shame on him...) but was quite reluctant to upgrade to a newer version in case the phone would be bricked.
Fortunately, I gave him a link to ZiPhone, which is the best way to unlock your iPhone I've found so far: just upgrade your phone using iTunes, launch the application and click. 5 minutes later, you have a jailbroken and unlocked iPhone with the latest firmware :-)
Dan has also blogged about Fring, a very nice application to to VoIP over the iPhone.
If you like to twitter, have a look at Twinkle too.