Indeed, doors make excellent build tables, particularly with some sort of "pinnable" material on them, as Missleman suggested. Personally, I've had good luck with "Homasote", the black/brown "sheething" stuff put on walls of houses under construction. Anything that'll stay flat, keep its shape, and let you stick pins in it will work fine.

One comment on the hollowed out door idea, however:

While they do, indeed, make good build tables, the hollow core doors will tend to sag a bit in the middle over time. While you may not SEE a 1/2" sag in the middle, it can produce some warped/bowed wings/fuse sides/etc in a hurry! [X(]

A few options here : The shortest term/least expensive option is simply to be aware of the potential, and doublecheck your building surface to make sure it's FLAT before starting on any critical component. If you find the door has sagged a bit, you can simply "prop it up" in the right location, or perhaps move it a bit to eliminate the low area. A longer term solution is to build a solid framework of some sort to put the door on, that will prevent it from bowing/sagging. The "longest term" most reliable solution, though a bit more $ on the front end, is to go with a "solid core" wood door. They're a bit more expensive but not outrageously so (I got a 3' wide solid core door for $42 at Lowe's several years ago), but they absolutely will NOT sag or bow, even if supported with just a couple of sawhorses. of course, they ARE a good bit heavier, so not a good option if your table needs to be mobile.

Again...nothing wrong with a hollow core door at all, just be aware of the potential for a surface that isn't flat, and have some plan in place to deal with it.