We have 3 trolleys with 16 laptops each. The AP's travel round with the trolleys and plug in in the classroom (fortunately we are in a nearly new building and flood wired). Using WEP at the moment but perhaps I should be looking at something more secure.
Don't underestimate the amount of technician time moving, supporting and maintaining them. I've estimated 30-50% of a technician's time during term and am recruiting as a result. Software installations / updates are a pain as you need to get them out of the trolley, find somewhere to plug them in to the network and power and put them back afterwards.
They are not a cheap option compared with a suite of desktops. I would have preferred the latter but it would have meant nicking half the library or building an extension!