RF interference can significantly hamper the operation of a wireless LAN. As a result, it?¡é?€??s very important to identify and minimize potential RF interference. You can view more details about minimizing RF interference in a tutorial I?¡é?€??ve posted at the following URL: http://www.wireless-nets.com/resources/tutorials/minimize_802.11_interference_issues.html
I agree that existing neighboring wireless LANs set to the same channel as yours can cause interference. The impact is relatively low (insignificant), however, when there is no data traffic on the neighboring networks. As data traffic increases on the neighboring network, capacity of your network decreases. This can cause big problems to networks supporting voice and higher end data applications.
We have to differentiate the "Neighbor WLAN Interference" category.
Co-channel: Isn't RF interference at all. Just traffic on the channel, no more, no less.
Adjacent channel: Definitely RF interference and a pain to mitigate without a visit to the neighbor's office (or use 5 GHz).