Number of recipients seems like a good idea for separating personal and business to me too. Personally, I only use PGP for emails with my partner, but we definitely send way in excess of 150 messages a month.

I'm all for a world where people start using encryption as a matter of course, so I really the OP's product works out. It'll finally get rid of that "you must have something to hide" mentality, when all you really want to do is protect yourself from the odd security vulnerability or a grumpy sys admin with an axe to grind!