I was recently listening to Sleep's Dopesmoker album, heh, actually the whole album is just one song 63 minutes long!
This is one minute sound sample, after the distortion comes out, the effect is clearly heard, sounds to me like clean guitar with.. hm.. leslie effect? It does remind me alot of some hammond keyboards or some similar key instrument used by pink floyd (on live on pompeii for instance...)...
http://www.diyuaudio.com/stuff/01-sleep--dopesmoker.ogg
That's probably one of those old poly-chorus things Kurt Cobain used to use. Sounds a lot like it at least. Check it out: http://www.ehx.com/ehx2/Default.asp?q=f&f=%2FCatalog%2F14%5FModulation%2F14%5FStereo%5FPolychorus
It does sound to me like the polychorus/echoflanger/polyflange, I have an original and it gets that sound (and a million more).
I published a Layout for the echoflanger if your interested, Its a huge build. 2XSAD1024A, its basically a dlx elec mistress with one of the best chorus effects ever made and a slapback delay.
Although a small Clone with a "tune" pot mod will get you pretty close.