My DS (now nearly 6mo) used to only sleep for 40 mins at a time. He would occasionally take longer naps - but usually only if I was cuddling him.
He will still only sleep for about 40mins (if that) in his cot, but if I put him on a beanbag in the living room he can sleep for up to two hours for a post-lunchtime nap. I hasten to add that I am in the same room if he's sleeping in the beanbag before you all tell me what a nutter I am!!
I think one of the reasons I continue to do this is that if he falls asleep while being cuddled or fed, then he'll often jerk awake if laid down in his cot but feel comforted by the beanbag and go back to sleep.
If he wakes up and seems happy, I'll get him up. If he wakes up and cries or otherwise seems unhappy, I'll cuddle and/or feed him and he'll go back to sleep for another hour.
My question is really this: am I helping him to develop the habit of a longer nap during the day by doing these things or just giving myself problems further down the road?