i use my laptop in lectures to take notes instead of a notebook - i can't read my writing, plus i can type almost faster than i can talk...
pbl - never taken my laptop there, because i don't take notes in pbl so much.
library - very very useful (for me) - i don't have to buy any textbooks, and i can make notes from textbooks straight onto my laptop.
great outdoors - tried that once, and got a leaf stuck in my fan
(windy day).
residential college - they work alright as a desktop replacement, but they're nothing compared with what a desktop really is like, although if you're like me and you like studying upside down, it's better to have a laptop because you can drag it around the room (although i guess with wireless keyboards and things you could do that... but i'd have to stick my monitor upside down above my bed or something)
train/bus/plane - 5 hours difference between my home town and uni means that yes, it's great for the train/bus - read notes, listen to music...watch grey's anatomy
the biggest difference between desktop and notebook is if you want to play games really. i would not recommend a laptop for games, because you can't really upgrade them and you'd want to buy a high end one ($5k) to last a year playing the latest games, whereas an equivalent desktop would be more like $2.5-$3k. but laptops are great for their mobility & versatility.
bicycle - i've never tried it, as most laptop backpacks are huuuuuuge and i tend to also stuff them with textbooks and i think i'd fall backwards off my bike. make sure you shop around for a backpack because i know you can get smaller ones, but they're harder to find (plus they're also quite often expensive, at least for a bag).