BuildCraft 2.2.1 has finally been released with SMP capabilities. Posting the changelog here. Enjoy!
#226 SMP pipe render client freeze
Problem related to freeze when using pipes in SMP should be fixed.
#239 increase oil quantity in medium deposits
Medium oil deposits are now twice as frequent, and contain slightly more oil.
#245 wrong icon in markers when traveling in pipes
Icon of markers when traveling in pipes have been fixed.
#249 port to SMP
BuildCraft have been (re) ported to SMP.
#253 opening autocrafting table in SMP kicks client
Auto-crafting table does crash anymore when openned from the client.
#264 liquids spawn next to tank if taken out
Liquids can now be correctly collected from tanks.
#265 stone engines burn animation broken
Stone engine fire animation has been fixed in the GUI.
#266 oil bucket config
Oil and fuel bucket configuration has been fixed.
#268 empty bucket in iron engine prevents cooling
Iron engine can now cool down even if they contain an empty bucket.
#271 used Forge ISidedInventory interfae
ISidedInventory interface is now used, enabling interaction between pipes and
Forge compatible inventories such as RedPower or IndustrialCraft.
#272 add liquids hook
Liquids hooks have been added, for sub-modding.
#275 iron pipes don’t bounce items
Iron pipe are able to bounce items again.
#276 oil generation is buggy
Cases of no oil generated, or too much oil, should be fixed.
#278 unstable wooden pipe source
Source of wooden pipes should not switch randomly anymore.