Is there currently an editor available for Linux platforms, so I could develop on linux for various platforms?
Presently there is no Linux binary distribution of the UE4 Editor, but you may build it yourself from the source code available on the GitHub. Also, the Editor is not production ready yet so your mileage may vary depending what you do.
Please check our Linux Support Wiki for instructions and any common troubleshooting issues at: A new, community-hosted Unreal Engine Wiki - Announcements - Unreal Engine Forums
Oh, and we have no tested (extensively) any of the other platforms (HTML5, Android) as the focus is on getting the Editor stable first.