This is a big part of what startups need to think about from the get go.
You can find offers on Craigslist and elsewhere for $500 jobs to clone a Meetup or a Groupon.
Typically it's the little companies copying the big ones, not the other way around.
But in the end, its cheaper to focus on execution than to head to court. Evite will make a lot more people aware of this market, and Paperless Post has the opportunity to steal them away.