holy crap i LOVE coaster brakes. have one on my 68 schwinn racer beater bike and i can bust the longest skids on that thing!!!
in fact a track bike with a coaster brake sounds fun.


anywho i will chime in about knee pain and fixed gear riding.....

get a front brake. unless you are doing "mad bar spinz" then i think theres no point in NOT having one.
oh yah, knee pain. OK, well first off i got into cycling in general for physical therapy for my left knee, which i have hurt many times playing soccer.

at first, when riding fixed for the first time ever, my knees hurt a bunch.....BUT it felt more like muscle soreness than damage. like, after going to the gym and lifting weights for the first time pain.

if you are new to riding fixed, it could be your muscles are just not used to that backwards peddling action. if you think about it, if you have only ridden freewheel, your legs are only peddling forward. BUT riding fixed introduces BACKWARDS peddling which your legs arn't used to. sort of like if you only did bicep curls and never worked your triceps, then one day decided to work the crap out of your triceps. your triceps would be like "WTF?" and hurt like a b!tch.

also though, skid stopping is much more forcefull backwards peddling....so you are putting allot more force on your joints. whereas gradually slowing yourself down by resisting the forward motion is probably good for you and helps work out those opposite leg muscles, the much more extreme force of skidding is probably kind of bad for the joints.

in a long winded way, what im saying is keep it easy on the skids, get a brake so you dont have to skid as much, and keep riding fixed to build up all the muscles in your legs.