is it technically possible to be a 2600 rated FM?

fide rules are not very clear.. but there are many restrictions other than rating, so i guess one could possible get a very high rating and still not be an IM?

Yep.

I know someone who had a 2550 ICCF rating and was not even a FM so certainly one could be a 2600 rated FM

that's too high! maybe he never played in an international tournament? or that's not a norm?

There is also this guy: http://en.wikipedia.org/wiki/Claude_Bloodgood

Apprently if they (1) let you play rated chess games in prison, (2) you are the best player in the prison, and (3) you are serving a life sentence you will wind up with a really high rating.

EDIT In case you wanted to call bullshit on some of the more outlandish claims in the wikipedia article, he really was convicted of escaping from prison when playing in a chess tournament after being convicted of murdering his mother. See Bloodgood v. Garraghty, 783 F.2d 470, 472 (4th Cir. 1986). 

Lol, Pfren your post made me laugh so hard, coffee came out of my nose.

I suppose it's possible. I remember hearing years ago that Gata Kamsky was the 3rd highest rated playef in the world before he got his GM title.

Not sure if it's true, though.