An F# Meetup Group in Seattle has now been launched by Tao Liu on the F# team. (It's only just been created, so get in early!)
If you're in the Seattle or King County area and are interested in F#, then please join, and help get the group off to a great start.
A good meetup group absolutely depends on people participating by giving good presentations, and also on getting F# presenters who are coming through town to come by and give a talk. So start considering what talks you'd like to give, and what other activities you'd like to see the group doing. The meetup group site is a great place to propose new activities, topics and speakers.
There are also other F# meetup groups in New York, London as well as other related F# or functional or .NET meetup groups