I have recently been involved in testing how the installation of a Zigbee controlled ESEL (Electronic Shelf Edge Labelling) system would affect the existing 2.4GHz infrastructure for a large supermarket chain. I can confirm AirMagnet's findings that when 'loaded' the Zigbee APs, of which there were two, had a severe impact on the existing system. To the extent that the client devices, which were transmitting and receiving a large amount of data, actually failed and lost wireless communication with the access points.
The original idea was to configure the two Zigbee channels to coincide with midpoints between channels 1 & 7 and 7 & 13 (we are in the UK), however although this appeared to work this was in a model store where there was nothing but the Wi-Fi infrastructure operating, ie it was a particularly 'clean' RF environment with no other interference. However it has been our experience of these stores that they are already overloaded on the 2.4GHz front with PIR systems being a particular problem, and if the Zigbee system was installed as well with a possible four APs and 20,000 nodes the bandwidth would be swamped. Consequently we have advised the client to avoid at all costs and utilise other ESEL systems that do not use the 2.4GHz bandwidth.