Clydesdales/Athenas (200+ lb / 91+ kg) - Clydesdale riding single speed mountain bike?

I'm 6'4" 275 and I get all my miles riding my road bike for commuting 11 miles each way 1-4 days per week. In the peak of my commuting season, which is starting this week due to the mild winter, I will drop down to 255 or so. I'm shopping for a mountain bike and I've been very intrigued by the single speed 29er's. I'm curious how single speed mountain bikes is for a big guy. I have a lot of mtb experience and have previously owned high end bikes but have only had a road bike for the past 5 years. I know most of this is determined by the riders fitness level and experience but I would still like to know your thoughts. Are single speeds only for those skinny guys you see cruising the city?

I don't know offroading, but I know singlespeed and fixed for hilly roads. I'm down to 235 from 255 - 260-ish, and I did about 20% of my 4500 miles last year on a singlespeed or fixed (mostly singlespeed, though).
What I can tell you is, without a doubt, one of two things will happen:
1) You'll lose weight and get strong or,
2) You'll get used to walking

That's pretty much it. You pick a gear that works fairly well for your terrain and grunt it out when it isn't great. If you ride a lot of flat, then your everyday gear will suck for climbing. If you ride a lot of hills (like me) then your everyday gear might top you out at only 19mph at 100rpm.

(edit)
No skinny girl jeans, though. Skintight nuthugger jeans are only for the skinny guys cruising around the city. :lol:

Lots of big guys ride SS mountain bikes around here. You didn't say where you live, so I can't say if it will work for your local terrain or not. If you are truly interested in it, go for it. It takes a different mindset than riding trails with gears. You need to be willing to do whatever it takes to conserve momentum, and even then you will be off the bike and walking sometimes. Most of the time you get to fly past all the guys climbing in their granny gears and give them guff about it. ;)

At your height definately get a 29er of some sort whether you decide on gears or not.

I'm 67 years old and weigh about 230 and ride 12-15,000 km a year on road and mountain. Last year I bought a Kona Big Unit 29er SS. Not many big hills in and around Montreal but I found that I had to use every opportunity to accelerate when it availed itself. Riding 3 or 4 days in a row was hurting my knees.
I converted the bike to MTB triple and a 9 speed cassette. Much, much easier on the knees.