Purplera1n, unrelated to the Dev Team, has announced a solution to jailbreak the iPhone 3GS for both Windows and Intel Mac computers. The jailbreak allows users to download Cydia on their iPhone to install unofficial applications on their device.
The jailbreaking solution provided by purplera1n still does not allow Winterboard to be used as of yet. The method installs an application called Freeze onto the device; this installs and unpacks Cydia.
Users who install Cydia onto their device can then proceed to install UltraSn0w which was recently released to unlock the iPhone 3G, now unlocks the iPhone 3GS. Users can freely use any service provider using a SIM card on their iPhone without being locked to selected service providers.
Users who have installed the iPhone 3.1 SDK will not be able to unlock their device due to the update in the baseband. The updated baseband in the 3.1 SDK prevents the exploit to unlock your iPhone from working. Users who updated previously may not be able to unlock their iPhone, according to The Dev Team Blog.
Warning: Users who jailbreak or unlock their iPhone may void the warranty and are cautioned to use this software at their own risk; purplera1n RC2 may still be buggy on Windows machines.