Update May 2009: A recent study in the U.K. found that, thanks to the recession, 5 million English homeowners have considered becoming 'reluctant landlords' by renting out rooms.
When my friend Michael, an inveterate couch surfer, turned up on my doorstep a few years ago, my boyfriend had just moved out, and I was wanting company and needing extra rent money. Thus Michael took up residence on the couch my ex had recently vacated, and I went about adapting to having a roommate who I didn't want to see me naked.
Aside from having to close doors and don a bathrobe more often than I was used to, my new living arrangement worked pretty well--for a while. Michael was paying rent and lending me a supportive shoulder. We had agreed when he moved in to check in with each other after a few months to make sure there were no major problems, and when we did so, there weren't. So Michael stayed on ... and on ... and on, and I commenced to kicking myself on a daily basis for not having set another check-in (or would that be check-out?) date. When he finally did move out after about a year, our friendship had been sorely tested.
The biggest lesson I learned? Sharing living expenses can put more cash in your pocket. However, If you ask someone to move into your spare room--or onto your couch--draw up a lease agreement, even/especially if your prospective boarder is someone you know. Have conversations about how to share living space, particularly if, as in my case, you only have one bathroom. Make sure to outline kitchen privileges, including whether or not you want to share food.
I was unprepared to be a landlady, and I wouldn't do it again in my current living space. If I ever move into a house with a granny unit, I might consider renting it out--as long as the unit has its own kitchen and bathroom.