"always free for 1 site" seems like a bad call. unless you have a ridiculous number of sites, is it really that much work to make a different account for each site, either with different emails or using a gmail/google apps email and putting a different +whatever in your address? I would guess that your rate of paying customers will be pretty low.
You make a good point and I've thought about that a lot but competitive sites offer free accounts so it would be hard for me not to have a similar option.
You could do something like zoneedit and just have some stern warning about dropping your accounts or charging you for back usage if you get caught circumventing the free limit by opening multiple accounts.
