highest you can go is 99, and yes it has been reached...it's possible to do it the legitimate way, or via glitch/exploit.
my highest ever was 65 on Kino (BO1) and so far in BO2 the best i've done is 26 on Tranzit, 28 on Bus Depot, 30 on Town and 35 on Farm.
Bus Depot was a fluke...our team of 4 managed to stay alive without perks that long on that stupid little map using the camping technique (3 players camp in the hidden corner, 1 player loops around the map gathering up the zombies for us to kill).
I got to level 42 on transit the other night. With a friend and two randoms who had never played before. After round 15 they had the hang of things and didn't need much coaching. We would have gone further, but we were a few hours in and I just wanted to go to bed. I do have a party planned out with 8 people, so we can play for an hour each on the 4 different systems and try to get as high as we can without getting too tired.
Will not change until the Lions win a Super Bowl. Hell, just make the playoffs 1-23-08