Radio Shack has a small plug converter which takes RCA to S-Video and vice-versa. There has to be some circuitry in there to do the conversion, but by the adapter size, it sure doesn't look like it.
I picked one up from RS about a year ago; it works great for my non-SVHS VCR.
Anyway, if you want a really nice switch, check out the Sima SVS-4 at
Buy.com. (It's what I'm using for PSX, Dreamcast, and SNES sound and video auto-switching to S-Video output). It's $149, but I've seen it as low as $80 online at times. Best Buy cleared em out last year at $80 also. 4 inputs, 2 outputs, composite -> S-video conversion, auto-switching, remote control, volume stabilization.