| I wonder about this too...I think in my case at least it's because my body is used to my bed, to the point where it doesn't feel too different from being in, say, my desk chair. Add to that the fact that I read in bed and have been known to node in bed (yeah Airport!) and my mind doesn't default to 'sleep time' when it's in bed.
In another person's bed or bedroom, however, the only referent my mind has for what's supposed to go on in there is 'sleep.' It's a bed or bedroom, right? There are no other associations like 'work on your paper' or 'do that report' or even 'grab a game of Q3 you slacker.' The only thing I know how to do in that place is sleep.
I find that getting a new bed tends to restore my ability to sleep in it for several months, even if the new mattress is worse. This is because it hasn't become associated with reading, noding, listening to music, etc.
Of course, in my case, the answer is simple...get a place of my own, with an office separate from the bedroom...make sure the only computer in my bedroom is my i-Opener for email and light surfing, and then...snoooooooz.
Alternatively, arrange with roommates to swap rooms every once in a while when I need to get to sleep. Hmm, that one holds promise...if only they weren't allergic to my ferrets... | 
