Greenpois0n iOS 4.1 jailbreak released, but only for Windows
This is a delayed release that's wrapped in controversy: Geohot, the creator of limeral1n, has apparently been a very naughty boy. Because of Geohot's antics -- he released his exploit too soon -- greenpois0n was forced to use limeral1n's iOS 4.1 USB boot ROM jailbreak exploit rather than their ace-in-the-hole SHAtter. Both jailbreak kits are Windows-only, with Linux and Mac support hopefully "coming soon."
Mac users can use Boot Camp, or another virtualization tool like VMware or VirtualBox, to install Windows -- and thus jailbreak their iOS 4.1 device.
[thanks to Gk for the tip!]