We would love to bill hourly. We tend to work for large companies, and they need to figure out how much to budget, so we then end up saying, "this will take between 10 and 12 hours," and then being forced to swallow the costs when it takes us 14. And of course, if it takes 8, they win. If anyone has suggestions other than "just tell them it will take what it takes," we'd really appreciate hearing them.
Edit: Usually, just stopping when the hours run out isn't a good choice either, because most of the projects we have won't provide a partial solution. It's all or nothing. Example: converting data between two formats. We discover a quality issue when we've run out of time, so the converted data is unusable.