We'll take guesses until we get a winner. The prize, grandly donated by myself, will be a custom leather dice bag made by Yours Truly.
(A quick pimping of my own site for examples of my work: http://www.rogueleather.com )
Da Rulez: one build per guesser, the first available guess is b44 (This was posted the day b43 was announced). Trevor is the unofficial judge (and therefore can't play, sorry Trevor), as he will be the one deciding whether or not MT is stable for release. (He may, however, be susceptible to bribes; you'll have to take that up with him

If your build number goes by the wayside (I.E., the next build is announced) you may guess again.
I will update this post as guesses are made. I ran the idea past Azhrei, and he got the first pick.
b44 - DEAD
b45 - DEAD - mrobviousjosh
b46 - DEAD - Azhrei
b47 - DEAD - CJack42
b48 - DEAD - plothos
b49 - DEAD - AidyBaby
b50 - DEAD - Orchard
b51 - DEAD - Lindharin
b52 - DEAD - Full Bleed
b53 - DEAD - RPMiller
b54 - DEAD - Mr. PokeyLope
b55 - DEAD - DrVesuvious (nice try on the +

b56 - DEAD - BigO
b57 - DEAD - mrobviousjosh
b58 - DEAD - demonnic
b59 - DEAD - Weero
b60 - DEAD - kat2cute
b61 - DEAD - Notsonoble
b62 - DEAD - Orchard
b63 - Azhrei
b64 - palmer
b65 - Brigand
b66 - RPMiller
b67 - IMarvinTPA
b68 - Witchlocke
b69 - Weero
b70 - Orchard
...
b73 - Silentstar
Good luck!
*Updated: added claims to above list. Tektonic, I'm afraid I can't honor your previous claim. Sorry

*Updated 11-17-2008 - b46 is current, mrobviousjosh loses on b45 and grabs b57
*Updated 12-29-2008 - b49 is current
*Updated 02-03-2009 - b50 released while I was distracted. Orchard is on deck.
*Updated 03-04-2009 - b51 was up than down, and b52 looks like it has a couple doosies. Full Bleed is up, but probably won't stay long.
*Updated 03-14-2009 - b53 is up, but probably won't make it. The end is getting near though! Added palmer at b64.
*Updated 04-02-2009 - b53 is down, b54 gives Mr.PokeyLope some hope, but Trevor has already mentioned fixing something for b55.
*Updated 04-18-2009 - b54 takes the long nap and b55 goes up.
*Updated 04-24-2009 (and waaay late) - b56 is out, but once again a new feature has raised a question of options.
*Updated 05-17-2009 - Added picture of completed prize!
*Updated 09-09-2009 - b56 down, b57 on deck.
*Updated 09-22-2009 - b57 bites it, b58 released and layout bugs found immediately. Will be replaced by b59 today or tomorrow.
*Updated 09-24-2009 - ...and b59 is up, but Trevor has again spoken of it's successor.
*Updated 10-26-2009 - again with the new build and probability of the next in one one page of posts. b60 is out, but some strange disconnected-client behavior hints to a b61.
*Updated 11-24-2009 - belated edits. b60 dead, b61 may as well be, we're pretty sure b62 will be it. Weero The Optimist claims b69.
*Updated 12-17-2009 - again with the late edits. b61 down, b62 was quite hopeful, but it too fell by the wayside. (and Orchard had that speech all planned out.) Now it seems that Azhrei is on deck again with b63, but Trevor has mentioned a b64 to solve a problem with the German translation file.