Update : The latest version of Redsn0w (Windows) can enable that tricky DFU mode on Windows! Macs can use the Pwnage tool to create the custom firmware itself…
Warning : This isn’t for the faint of heart. I did not post a detailed writeup cos’ DFU mode is kinda tricky on this one. If you know what this is and what it means, go ahead and try this. All others, please wait while other Windows users verify this.
The links are all interchangeable and you have all the mirrors you can ever think of! Use hj-split to join the files.
Activated iOS 4.1 Custom Firmware for 3GS iPhone [Official iPhone carriers]
Not-Activated iOS 4.1 Custom Firmware for 3GS iPhone [T-Mobile,other non-iPhone carriers]