I just found this quote over at runnersworld.com:
The cushioning of your shoe will compress while you're running. Most shoes take about 24-48 hours to get back to "full" cushion. Rotating shoes allows the cushion to "come back", shoes to dry after bad weather runs, and rotating different brands of shoes has been said to prevent overuse injuries.
How accurate is that - does rotating your shoes really increase mileage and/or prevent overuse injuries? I've heard people say they've got as much as 200 miles extra by doing that. I've just bought a new pair of Asics Gel 1160s, same as my other Gel 1160s save for the colour, and I want to know if I should use them now on rotation with the other shoes, or save them for when the first pair has had it?
Thanks in advance for any help. 
Edit: This makes interesting reading as well:
http://www.hillrunner.com/jim2/id191.html
Edited: 15/09/2012 at 16:54