Just figured I'd create this, as there has been an interest in the lack of information surrounding these OS versions here lately. Feel free to add to the list.

Released:
OS 4.3.0 - 8110, 8120, 8130 (Pearl 2 family)

Beta - Carrier Certification:
OS 4.3.1 / OS 4.5.0 (rebrand) - 8000 series (including 8700 family)
OS 4.3.2 / OS 4.6.0 (rebrand) - 8000 series (excluding 8700 family), 9000

Unreleased:
OS 5.0.0 - 8000 series (excluding 8700 family), 9000

Carrier certification for OS 4.3.1 / OS 4.5.0 has been underway since October, so you'd think that we'd have seen a release by now. With that said, February was the target date, to my knowledge, to coincide with BES 4.1 SP5. The new service pack has been given a release date of March 4 (partner availability) / March 11 (general availibilty), so the OS should follow shortly thereafter. I personally think carriers will be 'green-lighted' for release of OS 4.5.0 in the next couple weeks, but look for major carriers with decent quality control departments to hold out for a few more post-release bug fixes (likely a one to two month delay, in the March to May timeframe).

*EDIT: Due to the number of enhancements being added to the RIM OS, OS 4.3.1 was renamed to OS 4.5.0 for marketing purposes.

*EDIT: In the same rebranding fashion of OS 4.3.1, it's successor, OS 4.3.2, has also been rebranded as OS 4.6.0. This will make it's debut with the BlackBerry 9000, which I have predicted for a May announcement (at WES) and June release date (on Rogers, Vodafone, and AT&T, possibly others).