My poor dd has this and although it's not serious, it's really painful (mouthful of ulcers). I just wanted to remind mumsnetters of the symptoms so that they know when their children have it (not that there's too much you can do, but soothe them). It seems to be often misdiagnosed as either chicken pox or thrush by gp's.
It generally starts with a little fever and malaise. Then there are spots on the hands and feet and sometimes the bottom, and spots in the mouth that turn into ulcers. You can have the mouth ones without the hands and feet, or vice versa. The spots look just like chicken pox spots, but aren't generally on the torso which is where chicken pox usually starts. There is no "cure," but I've found a little nurofen and ice cream for meals (soothes the ulcers) helps. It's contagious in the begninning, but I think like most things, by the time you realize your child has it, they've infected everyone around. Adults can get it too, but not as common. My poor dd screeched with the first bite of her dinner tonight. Luckily I knew that 3 other children at the childminder's had hand foot and mouth, so realized that it was the food hurting her ulcers (found lots of them in her mouth that weren't there this morning), and not her being difficult. Anyway, hope this helps someone....
If you want to read more, there is info on this website