We've subscribed to two house swap sites over the past 6 months or so. The one that came through for us was Home4Exchange, although Homelink came close a number of times. I can recommend both of these sites, though I prefer Homelink: not because it was the one that finally delivered (this was luck as much as anything else) but because the website is marginally more usable.
The sad fact of the matter is that both sites are pretty poor. There is no such thing as a stored search (I had to keep entering the same old details every single time), and no attempt at automated searches or matching based on the many and detailed requirements that I entered about my own house and travel needs. Worse again: Neither of these sites provided RSS. I know - most people still aren't sure what RSS is all about, but on a site like this, notification of new potential matches would be a huge help.
Dunno if anyone out there is thinking of getting into this space, but it seems to me to be ripe for a shakeup. The idea of house-swapping is very (pains me to say this) Web 2.0 - there is already a more, how shall we say, earthy version, called Couch Surfing.