Back in March, we posted that we had started building nightly builds from mozilla-central/comm-central, but because the version of CentOS we had been using was too old, we were unable to continue providing Linux nightly builds. That has now changed and (as of today) we have both 32-bit and 64-bit Linux nightlies! Since this involved us installing a new operating system (CentOS 6.2) and tweaking some of the build configuration for Linux, please let us know if you see any issues! Additionally, some more up-to-date features that have been available in Mozilla Firefox for a while should now be available in Instantbird (e.g. dbus and pulse audio support) and even some minor bugs were fixed!
Sorry that this took so long, but go grab your updated copy now!