A few days ago, the unofficial iPhone Dev Team released an unlock for the iPhone 3G dubbed "yellowsn0w." The team subsequently released a 0.9.5 beta of the unlock app, which fixes several bugs in the initial iteration. However, many users are still experiencing issues, as noted in this.
Solutions for problems related to or caused by the update are beginning to materialize. A poster named "IphoneEntusiast" has published a step-by-step process for using yellowsn0w, from jailbreak through unlock. His process involves verifying that the Carrier field displays (null)(null) after QuickPwn is used, resetting network settings after installation of yellowsn0w and more. See the thread here.
Meanwhile, the Dev Team notes "there is a known issue with SIM cards that have STK (SIM Toolkit) application menus. These menus are usually items such as 'top up' 'get credit' 'lotto numbers' etc. These menu items confuse the application sometimes."