The BIAB USB outrigger drive has been working well with quite the majority of laptops, we even have great reports from those using the newer mini laptops such as the ACER or HP with it. Those little ~$200 jobbies. Small screen, maybe, but BIAB with RealInstruments on USB drive runs well on them.
I don't think you will encounter much in the way of problems, but if you are looking at buying a new lappy for the purpose, I'd say steer away from Vista OS and go for either Windows 7 or XP OS on it.
I would also recommend plenty of internal RAM. TIP: One can save money on new purchase if they are savvy and can use a small screwdriver, following directions. Laptops that come with lots of RAM preinstalled often cost more money than purchasing the same overall model with same OS that has less RAM in it and purchasing RAM upgrade stick(s) at the same time, installing them yourself as an upgrade. Or having the store install it can sometimes still come out as less overall expenditure.
Not to say that Vista is necesssarily "bad" -- but there have been problems and certain tweaks are necessary.
Also, Vista 64 bit won't run the VSC DXi software synth that ships with BIAB. That is because the VSC code was written 16 bit for use on 32 bit puters. The Forte DXi synth that pgmusic sells separately does work under Vista, but since we now have the Windows 7 choice, I'd go with that. Or XP in the right deal. XP is now the "mature" OS and I'm still using it for my music machines for just that reason!
--Mac