When I hear "liberal" I think of Ted Kennedy. More old timey Democrats, focused on bread and butter domestic issues and social equality.
When I hear "progressive" I think of Barbara Boxer and Russ Feingold - younger folks concerned about more modern issues, especially with more interest in foreign affairs.
The difference is very slight, but if you look closely enough there is something there. It's difficult to describe, but I know it when I see it.
There's also a difference electoral strategy wise in a sense. I would say Hillary is running the campaign of a liberal. Obama is running the campaign of a progressive.
Yup. Though Porce is right too.