Just a couple of days back, the iPhone Dev team had released RedSn0w 0.9.6 beta 1 which could jailbreak iOS 4.1 (and even iOS 4.2 beta) but only on iPhone 3G & iPod Touch 2G. Moreover RedSn0w 0.9.6 is still in beta and hence under testing and also available for Mac only.
Download Sn0wbreeze 2.0 to Jailbreak iOS 4.1 on iPhone 3GS
Download Sn0wbreeze 2.0 for Windows Update: Download Sn0wBreeze 2.0.1 which fixes the iTunes error 37 The detailed guide to jailbreak iOS 4.1 using Sn0wbreeze is coming up soon. Update; Guide to jailbreak iOS 4.1 using Sn0wbreeze