I think a lot depends on the size of your hands.
A lot of teachers advocate the short thumb (going back at least to Hogan's famous book). But, like Birdie_man, I've recently notice that I can't get much left wrist cock with the short thumb and it tends to put a lot of awkward pressure on the thumb to shorten it too much.
I have very small hands, however. Last weekend I picked up an old copy of Nicklaus's Golf My Way and I noticed that he has a whole seciton about how small his hands are and how this means he needs a longer thumb. It really opened my eyes, because Jack actually has a two page spread of just his hand prints (proving how tiny they are), and they are the same size as mine.
So I'm gradually starting to lengthen the thumb a bit, and I also wonder if there are downsides to this (Hogan worries about hooking and the long thumb, but Hogan always worried about hooking).
I have small hands too. (womanish even)
(and it means nothing BTW)
(no jokes please)
Coaks- i've found a longer thumb generally facilitates more downcock. if im not mistaken Hogan's thumb actually "lengthened" near the top of his backswing. Phil Mick has a long thumb and obviouly a lot of wrist cock/lag. Tiger has a fairly long thumb, but the interlocking grip prob reduces his wrist cock. it sounds to me like you want to lengthen your thumb, not shorten...
trying to create more lag and angle and have been playing with my left thumb making it shorter. Any thoughts on the thumb length and impact on swing?