My girlfriend has this problem, it's very weird to me. She's tried almost everything but she still can't swallow tablets. She has more success with capsules. I think it's actually the fear that is the problem.
I almost have the opposite 'problem'. I tend to swallow chunks of food that are too large, and often they'll get stuck in my oesophagus. It's a very strange feeling because you feel like you're choking and can't breathe, but you can still breathe.