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802.11b/g standards define channels that are spaced 5 MHz apart. In fact, each defined frequency is the center of a 22 MHz wide channel. In the US, channels 1 (2.401-2.423 GHz), 6 (2.426-2.448 GHz), and 11 (2.451-2.473 GHz) are said to be non-overlapping because they are separated far enough that interference between them is minimal. Some signal may stray beyond the 22 MHz channel width, but it must be at least -50 dB less than signal strength at the channel's center.
The draft 802.11n standard defines 20 MHz wide channels in both the 2.4 and 5 GHz bands, with an option to combine two 20 MHz channels into a single 40 MHz wide (control + extension) channel to increase throughput. There are several proposed methods for creating 40 MHz channels, and this issue must be resolved before finalizing the 802.11n standard. In particular, tests conducted on pre-802.11n products have shown interference between new products and existing b/g products and other-vendor pre-n products, so work remains to be done to ensure compatibility.
As the width of the band itself (2.4 to 2.4835 MHz) has not changed, there are only two non-overlapping (separated) 40 MHz channels in the 2.4 GHz band. Additional non-overlapping channels can be obtained by using 802.11n in the 5 GHz band. The current 802.11n draft specifies that High-Throughput (HT) APs should scan their environment before selecting channel parameters, and use methods to detect, deal with, and avoid use of overlapping channels.
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