Firefox 4.1 is not "out" - the current nightly builds are version 4.1a1pre, which means they come before the first alpha release of version 4.1.

Firefox 4.0 Release Candidate 1 has been built and will be published soon, and will hopefully be released as Firefox 4.0 soon. So the latest nightly builds are based on work that will eventually go into the next version. (We haven't actually decided whether the next mobile release will be called 4.1 or 5.0 or whatever.)

Nightly builds are development snapshots, not official releases. They always have version numbers that end in "pre." See my earlier comment about versions.