Or you could leave the vertical dropouts as they are and use a Trickstuff "Exzentriker" bottom bracket ?
It works exactly as it claims to, but there are a few things you have to be aware of. You have to use HT2 type cranks (or Race Face X-type etc - anything with a 24mm spindle) and certainly with Shimano XT cranks you'll need to remove the granny ring bosses for clearance. You could do this with a hacksaw and file - it's no big deal.
Also I believe that XTR HT2 cranks won't fit at all.....
The eccentric doesn't have enough "throw" to take up a whole link (1") so depending on what gear combinations and the chainstay length you might need a half link (I don't).
I'm not sure on pricing now (from www.trickstuff.de
) but the poor Pound to Euro rate at the minute will make it more expensive than when I got mine.
The beauty of it is that no frame mods are needed and you could always install it in another frame at any time, you keep the vertical dropout convenience for quick wheel removal and refitting and if you fit a disc mount it doesn't have to slide or be slotted.