Well, it was short. Forge 1.2.3 is out, and uncovers one BuildCraft severe bug that needs to be urgently fixed:
#113 pipe / engine drop issues
Pipe are properly dropped again, engine do not crash anymore.
BuildCraft 2.2.9 + Forge 1.2.2 works great AFAIK – so you only need to upgrade if you’re running on a server that uses Forge 1.2.3 (which is going to be the case for the BuildCraft official server anytime soon).