Download Squad says Sept 9th for FF3 for WM. Soon but not quite yet.
Well Firefox 3 is out and word is that Fennec ( the current name of Firefox mobile ) is rapidly being developed. Details can be found on the Mozilla mobile wiki, and if you are sufficiently brave you can download a test version here and be the first on your block with Mozilla Mobile. The first public release is expected late this summer ( 2008 ). It will be available for Linux and Windows Mobile 6 and up.
Firefox for WM is supposed to be released after Firefox 3 for desktops is out.