A couple of hours ago, Apple released new firmware for the iPhone and iPod Touch, bumping up the version number to 1.1.1. And they've made good on their promise warning about bricking iPhones.
WARNING: Apple has discovered that some of the unauthorized unlocking programs available on the Internet may cause irreparable damage to the iPhone's software. IF YOU HAVE MODIFIED YOUR IPHONE'S SOFTWARE, APPLYING THIS SOFTWARE UPDATE MAY RESULT IN YOUR IPHONE BECOMING PERMANENTLY INOPERABLE. Making unauthorized modifications to the software on your iPhone violates the iPhone software license agreement, and the inability to use your iPhone to unauthorized software modifications is not covered under your iPhone's warranty.
Gizmodo has all the gory details in a massive and regularly updated post, but to spare you the agony of reliving this nightmare over and over again, here's the quick summary.
Don't update your iPhone, period. Wait patiently for the iPhone Dev Team or someone else to release a workaround. anySim or iPhone Sim Free will fail upon updating the iPhone to the latest firmware, resulting in a locked iPhone. The iPhone isn't bricked, but you can't activate/unlock/use it either, so I'm not sure where that actually leaves you. The only exception to the rule is that iPhones unlocked with iPhoneSimFree are able to use AT&T sim cards. Which kinda defeats the purpose of unlocking.
If you remained unconvinced, Jonathan Seff @ Macworld tried it out and was left with an iBrick.
[Link: iPhone Firmware 1.1.1 Out and Tested: Breaks 3rd-party Apps, Relocks iPhones and Sends Them to Semi-Brick Activation Limbo (Updated with video) @ Gizmodo]
Edit: [Link: What does and doesn't work with iPhone 1.1.1 (Updated 9x, now with more bricking but custom ringtones) @ Ars Technica]
Update: How to Downgrade from 1.1.1 to 1.0.2 aka pulling a kmac from
***Update***: Bricking can be reversed with iPhoneSIMFree's new SimFree v1.6!