Channels are a myth
Ok, not exactly a myth but misunderstood.
All WiFi devices recieve over the whole WiFi spectrum but the signal gets weaker the further away from your channel it gets. Pretty much the same as increasing the distance or number of walls between stations.
In WDS mode you have to set the channel on both stations manually. Yes it works best if they are both set to the same channel but it still works if they are set to different channels, providing they are close enough.
WiFi devices themselves actually work quite well together but not everything using the 2.4GHz spectrum is careful not to trample another signal.
The chipset of the device is very important too. Atheros devices generally work a lot better than Broadcom in my experience. The Broadcom ones can get so bogged down with other peoples noise that they don't work. The Atheros ones seem to be able to use the frequencies that are operating.
WiFi is similar to ADSL in that it transmits on lots of little channels at the same time, some of which will fail and some will work.
Eventually people will stop using the old baby monitors as their children get bigger. They will also get fedup with video senders and get a cable put in. People should also put in CAT5.