The vast majority of projectors in use have a maximum resolution of either 800x600, or 1024x768, so make absolutely sure that your game runs at one of those.

High-definition projectors (i.e. 1280x720 or 1920x1080 resolution) are on the market, but I doubt that they have much of a market penetration yet, nor will do for the next few years.

It works technically just like a screen. Support for low resolutions is important. Mainly just try to run the game at the resolution that is allready in use, as it's most likely the preffered resolution of the screen.

make sure you design the game with the distance from screen in mind. consider using an xbox 360 usb controller or similar for gameplay, as keyboard and mouse might not have a good stand.

personally, i have a fullhd projector, an xbox 360 wireles controller, and rock old emulated games on it, now in hd :) (games i owned of course).

It is a good practice not to assume anything about the end-user resolutions in any case, regardless of the display technology. This reflects on the needed sharpness of the textures, pixel shader complexity and fidelity of geometry too. It is possible to query the system for supported output resolutions; this is a good practice instead of hardcoding resolutions.

In addition, be sure to test your game on low contrast settings (adjustable on most monitors) since the dynamic range of projectors is commonly very low as compared to LCD monitors. For example, do not assume that nearby colors can be distinguished as easily on a projector as on monitor.

Although the performance characteristics of a projector can be wildly different from a monitor, it generally acts as an ordinary display device from the computer's point of view so no extra coding is necessary to specifically accommodate usage of a projector as opposed to a monitor.

oh yeah, the colours. make sure you have big contrast in your game, something like resident evil (pre 5) with all it's dark muddy grey brown blackish can be quite hard to see. colourful, with bright parts and dark parts is best.

and not too many game-relevant details. people are far away from the screen, they most likely won't be able to see individual pixels or actually click them. expect inaccurate controls.

at best, implement some brightness settings in the game options (halflife2/portal has this option, as have tons of others). at best an adjustable gamma ramp, too.

Also make sure that your game plays smoothly at fairly low refresh rates.

LCDs tend to refresh at 60hz, and decent CRTs go all the way up 120hz, but your average projector from a couple of years ago is lucky to run at 30hz.