In past cons informal meetups were at "the bar" or a restraunt down the street or even in someone's hotel room. Often organized within the community of interest. Misti-con is formalizing this.
An example of an informal meetup is a typical con "shipper meetup".
What I beleive is by filling out the form, the meetup is put on the schedule versus word of mouth.
http://www.misti-con.org/meetups.html Of course this doesn't prevent folks from having informal meetups in their rooms.