As I’m sure many people have already noticed in the past, if we take an audio signal and combine it with a completely inverted version of itself, the two signals cancel each other out perfectly to create total silence. In other words: 1 + -1 = 0 (Makes total sense, but I was quite surprised the first time I actually tried it in a sample editor!)
You can test this very easily in Renoise by taking a sample and playing two identical notes on two separate tracks, but on the 2nd track you add a Gainer device and invert both the left and right channels.
If we now apply some kind of effect to one of the tracks, we get an interesting result that is the difference between the original signal and the effected version, but in a way that you might not expect at first. For example, if we add a lowpass filter to the 1st track, and leave the 2nd track in its original (but inverted) state, these two signals will now actually combine and create the opposite of a lowpass… a highpass! (Albeit a somewhat crude sounding highpass)
In Renoise, when using the Stereo Expander device, if we set the Expand parameter to “Mono”, Surround to “0%”, and Mono Mix to “[L + R]”, this will actually give us the mid/mono portion from the signal. I thought that fully expanded might also give us the side/stereo portion on its own (which would make this a whole lot easier), but this does not seem to be the case.
Anyway, so, my basic idea was that: signal - mid = side
I tidied up my original example a bit now, since I realised I could do some things slightly differently. It’s still kinda weird, but hopefully it’s a little bit clearer now, haha. In any case, the process of combining these elements and subtracting the bits we need is quite trivial in theory, it just requires some interesting send track and gainer device mathematics 
http://illformed.org/temp/2009-07-28-mid-side.xrns
[Edit]
Here’s another quick example using a clip from the song “Young Folks” by Peter Bjorn and John. The mid/side isolation technique works particularly well on this song, creating a rather nice vocal extraction effect when you mute the mid portion and leave only the side.
http://illformed.org/temp/2009-07-28-mid-side-2.xrns
Disclaimer: the “Young Folks” sample is obviously copyrighted and is just used here for demonstration purposes. Please be sensible.