I recently started using MIUI. Each time I installed the weekly update I would wipe the phone completely, install the update and flash the boot image (I have a HTC dev unlocked Desire S) and re-install everything. Ibotpeaches kindly pointed out that I didn't need to flash the boot image each week and could just update, keeping my settings, apps etc. I was wondering if the boot.img ever changes and if so how often and how would I know? Hope I make sense.