It's not unheard of to have a moratorium based non-compete (specific markets/market types) in the license.  Include support/availability for new features, etc. so they're likely to enjoy the license/throw more money your way and I'd go for it.  It'd really depend on how core to the business model the software is.  It'd be like anti-google, trying to start a business but end up making a business out of licensing your software.