What was your reason for giving up "coffee," specifically, and not giving up "caffeine"? I ask this because decaffinated coffee still has a small amount of caffeine. Chocolate has caffeine. If you're still drinking 3-5 cups of coffee per day, albeit decaf, you're still potentially consuming a fair amount of caffeine. Were you trying to give up caffeine completely or just cut back? Numbers can be found here: http://www.energyfiend.com/the-caffeine-database
I gave up caffeine--not coffee--in stages after I finally had to admit to myself that even a latte in the very early morning was affecting my sleep in subtle ways. I switched to decaf first, then switched completely to herb tea. Then I ceased my consumption of chocolate (sob). I didn't gain weight after eliminating caffeine, perhaps because I did it gradually, but I definitely felt hungrier for a little while (my latte served as a midmorning "snack" of sorts). I'd say it took about six weeks for my hunger levels to return to where they were with caffeine.
I wouldn't overestimate the effect that caffeine was previously having on your metabolism if I were you. The half life of a dose of caffeine in the body is about four hours, and estimates I've read of the increase say it would be a change of about 50 calories in that four hours (this link is a crappy source but it's consistent with more authoritative stuff: http://voices.yahoo.com/can-coffee-safely-raise-metabolism-weight-loss-10890739.html, I'll see if I can find a better one). That's not enough of a metabolism change to explain ten pounds in six weeks, particularly if you have not completely eliminated your caffeine intake because you are still drinking decaf. Parenthetically, I think it's about as hard to change one's metabolism as it is to build muscle--that is, it's really hard. Caffeine is highly prized in the endurance sports world for a boost before a hard workout or a competition, but the effects are not regarded as long-lasting. I do still miss that cup of coffee immediately before a six a.m. spinning class. Sigh. I have more sympathy for addicts now, talking about this makes me miss it.
More likely, your previous caffeinated coffee intake served as an appetite suppressant, and the ten pounds arrived when you stopped routinely suppressing your appetite.