As per Eladrin in the Ranger forum earlier (link = http://forums.ddo.com/showthread.php?t=160212), enhancement lines will follow one of two paths - either 6/9/12/15/18, or 6/12/18.
Still no news about the benefits and/or requirements for Kensai, PDK, and (Dwarven) Defender, but I think it's safe to say that the enhancements will eventually have 3 levels at the very least (possibly 5!). And if you want all of them, plan to go to 18 with them (i.e. maximum splash of 2 levels in something else).
Just a huge heads up for all you character builders out there.
(also a heads up for those with Rangers, Rogues, etc. Plan ahead!)