Trucco looked for Google searches about iPhones slowing up and found that searches on the subject spiked a few months before each new iPhone model was put on the market.
I experienced the slowing effect on my iPhone3G when I stupidly upgraded it to a new OS.
Desperate to reverse the upgrade I rang Apple who told me it was impossible. However the very nice Apple-ite I spoke to very kindly pointed me to a web-site which could help.
To my great relief it did, though it was a fairly complicated series of steps to do it.
Since then I have resisted all of the many attempts by Apple to get me to 'upgrade' the OS on my iPad2.
And it still works fine.