I had a laptop with 2 good batteries and I lasted out a power cut watching a whole DVD. The ADSL 'modem' was powered by the phone socket and shared over Wifi from another laptop with a (single) good battery, so I could continue using the internet while watching this DVD!
I guess what I'm trying to say is that laptops might be a good solution for a hosting company, because of the lower power requirements and cheap batteries.
That's not even remotely price feasible. Also, lower power requirements does not necessarily mean efficient. In terms of power/watt, they have inefficient power conversion and are particularly difficult to cool. Trust me, rackspace knows what they're doing.
Google actually implements batteries on their servers individually rather than as a UPS for many. The problem with "lower power requirements" is that it normally coincides with slower speeds/less efficiency in some fashion.
I guess what I'm trying to say is that laptops might be a good solution for a hosting company, because of the lower power requirements and cheap batteries.