I rebuilt the front wheel on my commute bike around an ebay sourced Shimano XT hub. It was the same dimensions as my failed shimano 600 front hub so I was able to re-use the spokes and rims. It's been flawless, and didn't seem to weigh much more if anything than the original 600 hub. I would expect a rear hub to have more of a weight difference between road/mountain, but that's just a guess.

I've got a brand new front/rear set of 2010 Tiagra hubs sitting in front of me on the desk that were given me by the LBS for compensation over a matter. I sure wish there was a way to sell/ship them to you affordably. =( They are very nice but I simply have no use for 36 hole hubs, and will end up off-loading them on craigslist or something.

-Jeremy