It probably varies by experience. I know I used to have trouble sleeping in unfamiliar places, but then there was a period of my life where I was moving, traveling, and visiting people a lot, and sleeping in a lot of guest beds or couches. It got a lot easier for me to sleep in new places after that.
Like jdimov9 I find that new places seem to make me sleep more soundly.
I did a fair amount of backpacking when I was younger (mostly in Scotland) which, almost by definition, includes sleeping in lots of different places - both in tents, bothies and sometimes bivouacs. I wonder if the association of being physically tired (carrying a pack all day up/down mountains) and new places caused an association with good sleep?
One odd association that I have is the sound of rain on an umbrella makes me drowsy - I've spent a lot of time sleeping in tents listening to rain!
