I confess I'm also curious about the Surface Pro for this application (and I'm a bit of a hater of the 'iPad' and all tablets in general, since they all seem to me to be more vehicles for consuming multimedia content vs. having much actual utility, outside of highly scripted cases...I do love my Kindle though, but that's just a digital BOOK and smaller than the 2-4 hardbacks I'd be lugging on any given week otherwise).
Would need either a handfull of really big micro SDHC cards for storage (it has a slot) or a thin external SSD...but the processor seems beefy enough, it's got 4 GB RAM and a good screen resolution for its size, and the included stylus would in my opinion be a plus for most Lightroom adjustments outside the keywording. The jelly keyboard makes it effectively a laptop if you really want to be 'typing' but doesn't add bulk, and the tablet form factor in general seems sturdier than the ultrabooks for travel slipped in a backpack or gear bag, has USB3 and a display-port for connecting to larger monitors...battery life is being predicted in the 4-5 hr range which is about on par with ultrabooks as well, although less than most 'tablets'... Price for the 128GB one is supposedly right at ultrabook $999 type levels but then you have to spring for the keyboard too.
At least it comes with a usable (for Lightroom, not turning this into a Mac-vs.-PC fight!) OS, vs. all the Mac-then-dual-boot suggestions
The biggest downside is you still need to tether to an external storage solution, whereas any ultrabook is going to have at least a 40GB or so SSD built in...
I confess on my last trip I ditched the laptop entirely and just spent an additional couple hundred to have a good SDHC card (8GB) for every dive day, swapping cards each and every day. I'd chimp and triage a few photos off the cards using the camera at night, plus gain my 'what should I try different next day' insight from that review, but told myself I wanted to enjoy my vacation vs. spending my evenings processing, as I knew I would if I had the laptop in hand. A pile of SD cards with a nice protective case is way smaller than the smallest laptop!
Surface Pro's aren't out until early next year though (don't get the Surface RT, although the formfactor is about the same if you were just curious as to its 'feel' its totally inapplicable for Lightroom), so you would have to wait if you wanted to go that route.