This is totally off-topic. In fact if off-topic is finite, then this wraps back to on-topic and right through it again back to off-topic. Twice. At least.
But maybe we have some wiz-kid here who knows about this, because I don't.
Basic situation: I have a HC4900 projector (which is a non-DLP) that produces polarized light. Now, I'd love to play some games and watch some movies in 3D and the resolution is definitely enough... even if I half it, I end up with 960x1080, which is plenty for most uses.
But I'd still need it to be 3D. Shutters are out of the question (too slow) and so are red/cyan images (awful image quality) and pyramid lenses/mirrors (can't align the image properly and besides, I'd need a fairly large patch). Which only leaves polarization.
Projecting half of the image onto the other half is simple (with a mirror), but because the light is polarized, I'd need to rotate polarization as well. and all I've seen so far are mirror systems which try to rotate each pixel individually, which is far to costly.
My knowledge of physics is pretty limited: Are there ways to rotate an image's polarization, without rotating the content. Maybe chemically? Or to randomize polarization with acceptable losses and heat?

I know some of physics side, but none of the technical side and how it is applied to 3D..
But yes, you can have linearly polarized light or circularly polarized light. That's about all I can contribute without doing some re-reading :)

Sounds like you're headed into the realm of James Bond villain science experiments ;-)
I think I know what you're asking, but I'm still a little hazy on what it's supposed to accomplish.
I'm guessing you want to project the left-eye image with horizontal polarization and the right-eye image with vertical polarization, or vice versa, yes? Then sort them out with polarized glasses?

Exactly. But for that I need the waves rotated and I've got positively no idea on how to do that (short of buying a DLP projector).
It's not quite as science-fictiony as it sounds. DLP projectors are capable of it and so the software side of things is actually looking quite well.

Basically I need to turn horizontally linearly polarized light to turn into vertically linearly polarized light :) The 3D part is easy: Project them on top of each other and put on polarized glasses.

Be aware of the following (from Wikipedia):
"The 3-D effect only works on a silver screen since it maintains polarization, whereas the scattering in a normal projection screen would void the effect."

Thanks... I didn't know that... now, how much do silver screens cost and where can I get one? (Just thinking aloud)

Exactly what are you trying to do here? Do you want to play Crysis in real 3D or what? All in all, I don't think you'll get around buying a DLP projector.

Probably not, but as I have a regular projector now, I wanted to know if this is possible. If it is, I'll do it. If not, I'll buy two DLPs the next time around, but not until my current one is finished.
For games: NVidia drivers have side-by-side stereoscopy built in, so in theory I could play any game. Movie-wise a bit of reformating would probably be required, but that's easy.
Looking at the prices for metalized screens, it seems like I'll land in the middle to upper four digit ranges.... which isn't what I want to spend right now, so most likely this will be postponed.

I must agree with you though, it would be WAY cool :-D

I just think that 3D is the next big thing that will happen in entertainment. First will be the cinemas, which try to get a head start while 3D is still too complex for home use. Then will be the handhelds, which can easily be converted for 3D with a cheap pyramid lense... the biggest problem is that you need a special one for each screen, but as there are relatively few handheld types (Nintendo DS, PSP, maybe iPhone) that hold a large part of the market share, this isn't really a problem. Once the cinemas and handhelds have been converted, people will start asking for home cinema support, and the industry will provide it. First through prefit LCD screens, then projectors.
So there should be plenty of content soon. And I want to use it.

It's definitely possible, but I've no idea of the practicality of the possible solutions.

Rotating plane polarized light is fairly straight forward process, as you simply have to pass the light through an optically active (chiral) material. The amount that the polarization is rotated depends on the optical rotation of the material being used, the distance you're passing the light through the material, and the wavelength of the light (this last one might make the engineering for what you're doing a little tricky). It's probably much simpler from an engineering perspective to just create orthogonal polarizations from the get go rather than trying to rotate the polarization of multiple wavelengths at once.