Inter-access point interference sometimes not a problem
The common design rule for installing 802.11b/g access points is to set adjacent ones to non-overlapping channels (1, 6, and 11). This avoids the access points from competing for the same part of the spectrum, such as when transmitting beacons, data frames, and acknowledgements. When analyzing existing wireless LANs, however, I often find adjacent access points set to the same channel. In addition, it's common to have neighboring wireless LANs set to the same channels as yours. For example, a quick scan of my residential area shows several homes having access points all set to channel 6, the typical vendor default channel.
I've been analyzing various wireless LAN issues recently and ran some throughput tests between a wireless client and a particular access point - with and without the presence of another nearby ("interfering") access point set to the same channel. In fact, I measured degradation of a few percent in throughput when the interfering access point was active as compared to when it was not. This indicates that there's not much impact from the beacons of a neighboring access point set to the same channel. The problem, however, is when multiple clients are utilizing the interfering access point. That can take away a large portion of the bandwidth on your network.