[michaelmartin]

I agree, but you need to be careful with the distinction between the paid and the ad-free versions.

Is it just removing ads? Or is it a premium product?

e.g. Gmail has no ads when you buy a Google apps account, but you pay $50 for it because they give you a load of other features as well (Which you may or may not need).

I still think it's good to give the option, but your selling points could get diluted pretty quickly once you start adding more little things to justify your paid memberships.