I think that using LED lights would work fine for growing pot. You just have to cover the absorption spectra of the plant. The only way, as far as I know, that the plant absorbs light energy is through chlorophyll a and b.

Looking at the absorption spectra chart Chlorophyll absorption spectrum It looks like there are peaks around 425, 460, 640, and 660 nanometers. LEDs are specified by their dominant wavelength...easy enough to select some that would work for that (or at least close to it...they only produce them at certain frequencies after all). As long as these peaks are covered, the plant will gather energy, probably not as much as from a full spectrum light, but the light that strikes the plant will be more efficiently turned into energy for the plant to use. HID lights do the same thing, but there are a lot of wasted frequencies emitted that the plant uses very inefficiently. Check out this link if you want to learn more than you ever cared to about light emission spectra: The Science of Color, the Emission Spectra of the Elements and Some Lamp Engineering Applications

Ok...so what about the brightness of LED lights as compared to HID lighting? It is true that most LED lights are not very bright (~.5 lumens each), but certain types are produced (do a search for superflux LEDs on ebay) that put out ~40 lumens each. Thats pretty good when you consider that an array of 50 puts out about the same as a 100W flourescent bulb (~2000 lumens). Yes, thats not nearly as much as, say a 400W HPS (~58000 lumens), but if heat and power consumption are a concern, then LEDs may be a viable option.

I built two little LED arrays out of the superflux LEDs I talked about and currently have a few seedlings underneath them. They are at least not dying! I'll upload some pictures soon. My aim is not really to have a large yeild necessarily, but to reduce heat and power consumption...and to try something cool. They were pretty cheap to build by the way...all it consists of is 2x $10 packs of LEDs, some perfboard (~$10), some reflectors and a DC supply (from an old computer....dedicated DC power supplies are pretty expensive) I jacked from work. It did take a while to wrap the wires however. Who knows...I haven't done a lot of growing and chances are I'll kill my plants without seeing how well my LEDs work!