I specifically asked my doctor about this because I was really liking sleeping on my back, too. My doctor told me that once I hit 20 weeks, I needed to be sleeping on my sides, primarily my left side. Up until that point, I didn't worry about it too much. However, it just gets more and more uncomfortable to sleep on your side, and sooner or later you're going to have to get used to it... so in a way you might as well start now.
I have REALLY bad hip pain now that I'm further along... I wake up constantly and have to flip over to help with that. I've found that propping myself up on pillows helps, so that I'm not directly on my side, but leaning more towards my back or my stomach.... but still not lying flat on my back.