I was reading "802.11 Wireless Networks" (Matthew S Gast, O'Reilly, ISBN 0-596-00183-5) and stumbled upon the fact that increasing the Beacon Interval on an Access Point (AP) can be a good thing. Firstly, the default setting on my AP was 50ms which means all my Wifi enabled kit in the house is being interrupted 20 times a second, which is a tad excessive. By increasing the interval to 1000ms my laptop is interrupted less frequently and hence saves some power. Secondly, increasing the beacon interval means I get a little more available channel capacity for my data.
The downside is that passive scans of the network take a little longer and also mobile devices on my network cannot move about so rapidly while maintaining network capacity - but this is not a big deal for me since my laptops generally stay fixed on my desk, dining room table or lap.
So now I've tweaked this setting powertop is reporting that iwl3945 wifi interrupts have dropped significantly from ~20/sec to ~1/sec on a idle system.
The beacon intervals section of the Less Watts Wifi power management page also concurs with this as being a good way to reduce power consumption. Scale this up in an office environment when tens of machines are connected to an AP, and you start to see some valid reasons for making this change to the default setting.