The Epic Games Forums have word not to expect an editor for Bulletstorm when Epic's and People Can Fly's upcoming first-person shooter is released (thanks BeyondUnreal). Word is:

Hey folks, sorry it took a while to get back to you here.

In Bulletstorm, we would have had to make modifications to the Unreal Editor to account for the skillshot system. We spent years developing the skillshot system and carefully balancing weapons, and it was a resource matter - make an editor we can ship with the game or keep polishing Bulletstorm for release.

Epic maintains the Unreal Development Kit (UDK) for people who would like to work with our game development toolset. We regularly release free, updated versions that include the latest additions to the engine. UDK *is* Unreal Engine 3 - plus it comes with sample content that can be used to make standalone games. Epic's engine team makes hundreds of improvements to the engine every month and UDK is the best way for developers to keep on top of the latest technology. UDK ships with the gorgeous visual features seen in Bulletstorm - Unreal Lightmass global illumination, light shafts (a.k.a. god beams), dynamic lighting and shadowing, and so much more.