Geometry:
I built up my fixie as a commuter/off season trainer from an early 80's lugged steel 61 cm road frame. My road bike is (sorry, was, as it was recently stolen) 58 and my tri 55. I prefer a lot of drop on the tri bike and a bit of drop on my roadies. I got the frame cheap, which is why I chose this size. You can go with whatever you feel comfortable riding, but most prefer road geometry. You can always change stem length to adjust the stretch. Stiffness isn't as important as in a track bike, as you'll likely never be sprinting on this.

Gearing:
It's up to you as to what you want to do with the set-up of this. If you have a 42 ring sitting around and want the gear inches with a 16 on the back, go ahead. Personally, I think the 42x16 is a little on the short side, and if you have any downhills, you'll be struggling to keep up cadence. I've hit 155 before my legs refused to spin any faster, lost the chain off the rear cog, and locked up the back wheel at about 40 mph. I ride 46 x 15 around Chicago and don't have a problem. If you're running without brakes, though, I find it easier to stop with a smaller set-up.
The only real differences in big/big or small/small will be stiffness in the chain ring (smaller will have less flex), and wether you'll be able to get your chain around both if you have a big/big (most fixies use BMX chains which only have 110-114 links--with a large frame a single chain may not be long enough). If you're building this yourself, ensure you have a narrow BB to get a proper chainline. You may need to put the chainring inside the crank splines for this.

Drivetrain Care:
This is the best part. You don't really need to do anything. Get a 1/8" ring, cog and chain - they're a bit stronger than 3/32" - clean and lube it when needed and you're good.

Hopefully this isn't too much of a threadjack, but jeez how do y'all know all of this fixie stuff? I am definitely interested in getting into the bike commute (easy as pie in NYC), but I wouldn't know where to start. It sounds like you guys know a ton about gearing and parts.

Shorten the cranks. You need better cornering clearance as you'll need to pedal around turns. I race on 175's but my fixie has 165's. Same Look Keo pedals so I only need one pair of shoes for the roadie and fixie. Just make sure whatever pedals you put on there have good ground clearance and don't lean as far into turns as you would on a switchback at 50mph.

As for the sealed hubs, most fixed/fixed or fixed/free hubs will be sealed. High end track hubs, maybe not. I know a ton of people who ride on formula hubs and don't have problems. I just built up a set of low flange IRO hubs I got from their bargain section for $30. Super smooth so far. The sealed BB will be similar. Any standard road, square taper will be this way. I have the shimano UN54, picked up locally for about $25 new.

WillbIM: Starting a fixie is easy. Just start stripping parts off your bike and don't stop until removing something else prevents you from going forward. Then get a new back hub threaded for a cog and lock ring. You're good to go.

it was more of an invitation than a warning :)

All depends where you get the parts and how fancy/expensive the fixie is to be. A reasonable price should be around 100-200$ if you make it yourself. But again, how fancy do you want it? Price out all the parts seperately. Use what ever you can from the old bike.

Re cranks tri-ac: I agree that shorter are better and DEFINITELY do not corner as hard. It could be painful (LOL!) Also, if you are using an older frame or non "velo" frame then shorter is better. Real velo frames are higher off the ground to account for the crank length. So be aware of that.

Best thing to do is to just go and look at real velodrome bikes and take digital pics of them, then decide what you want. Here is a great site that might prove useful. But then again all bike porn is useful!

http://fixedgeargallery.com/

:D

When I was a schoolboy in England, I busted my derailleur and couldn't afford another one. I happened to have a fixed cog on the opposite side to the freewheel.
So I flipped it.
I shortened the chain using a hammer and nail to break the link by pushing the pin out between a crack in the pavement and then re-connected it leaving out 8" or so. I squeezed the pin back in with pliers.

That was it. I rode it to and from school for a year or so after that until I got a job and bought another derailleur.

Never had a problem with it.

PoC

Here it is:
http://triac.files.wordpress.com/2008/08/rush-sm.jpg

I'll probably write a few comments about getting up to speed on it in a week or so...just gonna ride it right now!