Mobile builds are done on Linux ARM, inside of Scratchbox. For more details about the Mobile platform setup, see the ref platform doc. Builds are not yet being published, but we hope to get them going soon. Details are in bug 418852.
We have very very basic Mozilla2 infrastructure up. Linux and Windows builds are going, Mac is pending. Currently, actionmonkey and mozilla-central are built. There is no unit or perf testing as of yet. You can find these builds here and here. These builds are labeled as Firefox 4.0a1pre to avoid confusion with the current CVS trunk but that's the only reason they are named as such. We are a ways off from Firefox 4 and these builds should not be considered as previews of it. I've seen some blog posts about "Firefox 4 Alpha 1" being released and I want to stress that this is NOT the case. The 'pre' in the version strings means 'prerelease' - aka. not yet released. For all of the nitty gritty details on Moz2 builds, see this tracking bug.
Last but not least, Linux 64-bit builds have been set-up. They are built on CentOS 5.0 with GCC 4.2.3 and follow the same mozconfig as 32-bit dep/nightly builds. You can see them on the MozillaExperimental tinderbox for now, but they should be pushed to the Firefox tree by the end of the week. You can find the hourly builds here and the nightly builds here (the same place as 32-bit Firefox builds). These builds are being done primarily to "catch bugs in visibility handling" and make sure we don't break x86_64. At this time, there is no plans to do 64-bit releases.