While I'm not claiming to speak for all library employees, I worked for a public library for about six years* , including some time when wireless internet access was relatively novel. Someone camping out with a laptop wasn't a problem, even all day, provided they weren't being noisy (cell phones!), downloading porn, or otherwise being a nuisance. Actually, people who bring their own computers are probably a lot less trouble for staff, who may be busy keeping of track of whose turn it is next on the public computers, helping adults figure out how to file taxes online, etc.

The staff (and/or branch policy) ultimately decide what to do if a group of people is obviously using the library as a kind of temporary office, and they may request you make more formal arrangements. (The branches I worked at always had back rooms that could be rented out to tutors.) With several people, noise may be an issue. Still, one person typing, or two or three people working with occasional quiet collaboration is probably on par with a couple teenagers studying together. Just be considerate.

Libraries are paid for with tax dollars, and (as tptacek noted) staff are generally more nervous about empty libraries. Branch funding is often closely tied to usage stats - people coming through the door each day, circulation, internet station sessions, etc.

Also, depending on the neighborhood, the staff are probably more worried about drunk and/or mentally ill street people, misbehaving teenagers, etc. (Oh, the stories...)

* Great job to have through college, BTW.