lecturers don't care what you do in lectures as long as you don't talk. If you're going to distract other people, don't bother going (they're not compulsory).
As for laptops- i think it depends on the university and/or the course you are doing as to whether they are common. USYD Law, i would say anywhere between 50-75% use laptops in lectures now- people tend to type faster than they write, and in law most people just want to get as much down that the lecturer says as possible, then try to understand it later.
Arts subjects, a lot less people use laptops, although it is slowly rising. Last year I saw about four or so maybe using laptops (in a lecture of 60-150 people).
Science I would assume even less because it would be a lot more difficult to type up equations/diagrams (although I don't do science, so this is more a guess).
Battery life tends to be the killer for using laptops- there aren't generally many power points in lecture theatres (in the new law building at usyd they hardly included any in lecture theatres which is silly- although apparently deliberate because they think cords going everywhere is a safety issue or something stupid like that).
I use my laptop in my law classes, and handwrite in arts. The thing to remember is that you still have to handwrite exams- so keep up your writing skills!
