so many factors to think about...
1. minimum specs - you'll want at least a 400mhz processor, 128 mb ram, and at least 6 gb free space on your drive for recording and editing...any new laptop far exceeds these requirements, i'd recommend getting a 20-40gb hard drive so you can do multiple shows without having to offload all your data each time. a 24/48 recording will be about 1hr = 1gb, double that for 24/96.
2. size - how much do you want to carry and leave sitting out at the show? smaller footprints are better for taping as they leave less to be stepped/spilled on. also smaller screens use up less battery power although if you have it closed most of the time it won't make much difference
3. power - most internal batteries are not enough to last through a show, except for models like the sony picturebooks or fujitsu lifebooks (both are really well suited for taping) that you can get extended capacity batteries for. otherwise you'll need to get a laptop with hot-swappable batteries (ie. 2 bays that batteries can be inserted into) and multiple batteries or an external battery like the valence n-charge (my battery of choice) or electrovaya powerpad.
4. ports - smaller laptops may be lacking in some connections, or older models may suffer from this too. depending on the interface you choose to run with it, you'll need either pcmcia, usb, or firewire. pcmcia will be on every laptop. some older laptops may not have a usb port, but most do. many newer laptops, especially by sony, will have a 4-pin firewire port on it which you'd have to get a 4-pin>6-pin adaptor to run a firewire audio device on it...not too many laptop have full sized 6-pin firewire ports, though a pcmcia card adaptor can always be purchased to give you this capability.