If you do some searches on here and on google, you will find a good amount of info. A lot of people say you will have problems with fading with dyed upholstery, however I havent really seen anyone back that up with pictures.
I recently purchesed a set of brand new OEM seat covers that were taken off by the dealer for a leather installation. I picked them up for realllly cheap, but they were a light, pasty blue-grey (I think the interior color from the dark blue 4th gens). My plan was to do a good dye job on them and instal them right before selling the car to increase the value (although, now I have a set of new OEM floor mats as well, and the thought of an "all-new" interior is pretty appealing).
I picked up a few cans of black spray dye from the local auto parts store, found on the spray paint shelf. It's intended for vinyl and fabirc, and runs ~$3 per can. I layed a few good coats on the rear set covers and they came out nice and black, and then did the small vinyl patched around the edges with similar results. Then I threw them in the washing machine on "heavy wash" to test the color-fastness and try and break them in (the dye tends to make the fabric not quite as soft, a little "scratchy"). The dye held up well through the wash and spin cycle, with only parts of the vinyl needing some touch-up. Having the fabric OFF the seats for an operation like this certainly makes life easier.
That was about three months ago, before I left for my study abroad trip. I havent done anything with them since and never got around to installing them, they are still boxed up in my room waiting for me to have some free time. I might finish them up in the next few weeks once I start work and have free time in the evenings, and I'll get some pics up. Of course, if you are realllllly interested in this project, you could always just make me an offer . . .