I don't think that's a good solution for a lot of cases. When a service is free and everyone gets the same thing, something usually has to change if you're going to get anything for it. It doesn't have to be a lot -- could just be to remove advertising or offer one or two extra-but-useful features -- but I don't get the sense that most users will give their money to get the exact same thing (for Web-based services, at least).

If your service is always free and you're looking to benefit from the people who are willing to pay to get the same thing -- even though they don't have to -- then TipJoy is a good solution.