This should be an option that could be turned off if the premium user wishes..

It should also be set that if the premium user is following the free user the mention would go through normally.

It's an unlisted phone number except people can't even call you if you give it to them!

There goes any incentive for a company to set up a twitter account to interact with their customers who don't want to pony up the yearly fee.

I like being able to tweet @some_company and get a faster response than I would by filing a ticket.