I don't suppose you have Build: 4.08.605.15 710RD? The same thing is happening to me in that unrevoked is failing to flash despite rechecking all the hashes and what not. I've read that some other people who updated to the new build have also been unsuccessful in rooting using unrevoked but I have yet to try other methods. 

I've just started reading learning about rooting in general, so I have zero experience with this. Perhaps someone with a little more knowledge could chime in with a more effective way to get around this new build.