This is the deal (and see other threads for more of the same):

A 120 spaced rear hub has a 10 mm round axle and 120 mm spacing between the inside surfaces of the stay-ends (unless it was spaced 123 mm to allow for two NJS chain tugs mounted on the inside of the stay-ends).

The hub shells are identical between 120 and 120 hubs, but a 110 spaced rear hub has a 10 mm round axle that has been flatted on two opposing sides, reducing its dimension to 8 mm at the thinnest part (but 10 mm at the widest). How it achieves the 110 spacing depends on the brand: A Dura Ace 7600 high flange track hub has different cones and locknuts that take off 2.5 mm on each cone and each locknut (for a total of 10 mm difference) relatively to the 120 mm version. An NJS 120 mm spaced rear hub typically does not have a large spacer on each side, so you are often faced with having to replace not just the axle but the rest of the hardware as well (at least the track nuts are the same). Axle sets are available in Japan if you have the right sources but they're rare as heck here in the US, and without a machine shop you wouldn't be able to convert 120 cones and nuts to the 110 versions. In short, a 110 frame only works if you convert it to 120 or if you have a 110 hub in hand.

Converting a 110-spaced frame requires not just respacing the rear ends to 120 or 123 (which you can do with arm strength alone -- no tools required). It also requires increasing the spacing in the stay-ends from 8 to 10 mm (and in front, from 8 to 9). You can do a nice professional job of this (just like the Japanese frame shops do when converting keirin frames) with what's called a draw-file. this file doesn't have any teeth on the edges and has teeth only going one direction on each flat side (it looks like diagonal lines rather than crosshatching). A draw-file gives you a very smooth, finished surface and doesn't dig into the corners of the slot, and results in an extra 2 mm that looks as smooth as the original stay-end. Then use a non-tapered round file (a chainsaw chain sharpening file works just great) to smooth the front end of the slot. It takes about 20 minutes of vigorous filing to get it right, but it looks as good as any Bridgeport mill will ever do. The same draw-file is also good for removing burs and galling on the sides of the stay-ends where the track nuts muck up the metal (it's a cheap file, by the way -- good ones should run $7 or 8 for a 12-inch long one at an industrial supply house).

On the respacing, just push hard on one chain stay and pull on the other. Don't put all the pressure on the stay-ends themselves, or you are likely to bend them out of parallel, which will cause your hubs to bind. Do it a bit at a time -- you'll see the spacing increase by 3-4 mm with each moderate effort. After you've got the right spacing, you want to be sure you gained the extra millimeters equally from each side, so that the bike tracks straight. To do this, the easiest way is to loop a piece of string from one stay end up around the head tube and back to the other stay end, making sure that the path for the string is identical on both sides. Make sure it's taut, then measure the distance from the string to the seat tube directly between the outward bound and returning segments of the string. The distance should be the same on both sides.