I received the official OTA Google update to Android Ice Cream Sandwich 4.0.3 on my HTC Sensation. Every time now when I reboot the device, the system displays a notice stating that it is optimizing the installed apps. This should be just on the first reboot after the update and not every time?
This seems like it may be a bug in the update that HTC has released for their phones. It's been reported on the Android issue tracker and appears to affect various HTC models only. You can star the issue if you want, and some people have suggested that enabling the "Fast Boot" option in your settings may help (something like
Settings->Power, I believe). On other devices it should only run at first boot or when the Dalvik cache has been erased.
If it's of interest, the details regarding what the optimization is actually doing have been covered in What kind of app optimizations do newer Android versions do at the first reboot?