A few random technical questions...
 

(1) How are handlebars made? Do they start with a piece of metal pipe and bend it to a precise shape, and if so, how do they relieve the stresses introduced by bending? How do they make the clamp part of the handlebar thicker? On old handlebars, it's obviously a sleeve added on to the basic shape, but what about newer bars which just bulge smoothly in the clamp area?

(2) Why are spoke nipples made of brass? All other highly-stressed threaded fasteners are steel on a "typical" bike, and almost everything else is aluminum. So why brass for spoke nipples? My best guess is that it has to do with corrosion in some way.

(3) How do they put cartridge bottom brackets together? I spent a while staring at a Shimano UN72, and for the life of me, I can't see any seams or joints on the shell. On the other hand, on the Nashbar cartridge BBs, the bearing cartridges are clearly press-fit onto the outside of the shell.

(4) Is there any way to repair inner tubes with cuts right next to the valve stem? I have about 10 of these, and would love to know if there is a way to fix them...

(5) What kind of wrench fits a four-notch track lockring? I own a Park lockring hook wrench, and neither end fits a track lockring. I used the old mallet-and-punch trick to tighten it down.

I can answer #5, about the four-notch lockring. You use a bottom bracket that only engages one notch, but pulls in a very "clean" sort of way. Much better than hammer-and-screwdriver/punch method, which I've also used in the past. The right side of the pictured Park Tool is what you'd use for a four-notch lockring:
http://www.parktool.com/images/produ...5630_79816.jpg

Some of the other questions (all good ones) I have some inkling but am not sure, and it's late.

Yep, that's the same Park wrench I've got! I tried it for a 4th time and... ... yes, it does work after all *sheepish grin* Thanks Tim!

Small parts that require a lot of machining are frequently made of brass. For nipples it's easier to cut the threads. (For spokes the threads are rolled). The stress issue isn't as bad as it appears: the internally threaded part is usually softer than the screw it's meant to mate with, so the stresses are spread out more evenly.

(1) How are handlebars made? Do they start with a piece of metal pipe and bend it to a precise shape, and if so, how do they relieve the stresses introduced by bending? How do they make the clamp part of the handlebar thicker? On old handlebars, it's obviously a sleeve added on to the basic shape, but what about newer bars which just bulge smoothly in the clamp area?

Some bars are heat-treated to releive stresses, most aren't. Bar clamp areas are often hydroformed (think blowing up a balloon) inside a bulged mold. More expensive handlebars are sometimes 'drawn' leaving a thicker section in the higher-stressed middle and thinner elsewhere. Hydroforming tends to do the opposite (think about that when using cheaper thin-wall bars).

(2) Why are spoke nipples made of brass? All other highly-stressed threaded fasteners are steel on a "typical" bike, and almost everything else is aluminum. So why brass for spoke nipples? My best guess is that it has to do with corrosion in some way.

Brass is a low-friction material, plus it tends not to corrode too much.

(3) How do they put cartridge bottom brackets together? I spent a while staring at a Shimano UN72, and for the life of me, I can't see any seams or joints on the shell. On the other hand, on the Nashbar cartridge BBs, the bearing cartridges are clearly press-fit onto the outside of the shell.

Press-fit together by big machines.

(4) Is there any way to repair inner tubes with cuts right next to the valve stem? I have about 10 of these, and would love to know if there is a way to fix them...

No reasonable way to fix them, although I've got a couple of 1950's British cycle mechanic handbooks that show how to remove the entire valve and replace it with one from another tube (not much money around then, I guess). Deburr your rims and change your rim tape to stop that happening.

3) The internals (spindle and bearings) of a Shimano cartridge bb seem to be loaded from the drive side and then a friction fit metal ring is pressed in to hold everything in place. Look inside the drive side end, just inside the ring of splines for the remover tool. You will see a very thin metal collar that I believe is the pressed-in retaining ring.

4) None that I know of. You now have 10 circular bungie cords.

bars are butted externally on a mandrel, would be my argument, then bent to form. hydroforming is pretty new to the bike world, but bulged bars arnt, really. read all about mandrels and thier many uses:
http://en.wikipedia.org/wiki/Mandrel

also, paul makes a super dope lock ring wrench.

3TTT used to make their cheap bars by hydroforming decades ago. Their expensive ones were drawn over a mandrel, their intermediate bars were sleeved.

Interesting, interesting! Thanks for all the responses. I'll find some artistic use for all these old inner tubes, no doubt. When one of my Shimano BBs finally wears out, I'm gonna take it apart, if it's the last darn thing I do!

For #1, nitto sells some heat treated versions of their bullhorns for about twice the price of the non-heat treated. I've never used the non-heat-treated version, but my heat treated rb-018 bars are incredibly stiff. The stresses of forming a bar can actually make the bar stiffer. It will also be a bit more brittle, but actually stiffer and harder to bend. Its why if you bend a piece of steel/brass/copper its easy to bend but really hard to straighten. The process is usually called cold working.