From an 3d design (virtual) perspective, it's possible. The challenge is making the parts large enough to survive the wear. I personally am designing stuff that is very, very small. Operating gears are only a dream for the future for me.
Are you wanting to design things that do real functional work, or do you want to design smaller stuff that moves and "looks cool".
There are design rules that specify a "minimum wall thickness" for each material, but you're going to have to design things considerably thicker if you want them to not break under load.
An example is FUD.. it can survive printing at a thickness of 0.3mm but pretty much if you breathe on it, it's going to break. However, I've got one FUD model that is nearly 2.0mm thick, and I'd think that it'd support 5 pounds or more of pressure.
Take a look at this model
http://shpws.me/3VfA. That's a quarter there in the background. The gears you see are not even 3mm in diameter, and the teeth are at 0.3mm thick. Even though I have the capability of making the gears actually rotate, they first time they encounter ANY pressure, the teeth are going to snap right off.