Can an android app cause my phone to reboot? I have a Samsung Galaxy S (i500) running Android 2.3.5
Lately it has been rebooting randomly. 
Sitting on my desk and it just reboots, sometimes when I'm on the phone, etc.

Does Android protect itself from installed apps?
I thought the security sandbox apps run in was supposed to prevent an app from causing the entire phone to reboot.

My phone provider wants to blame apps for the phone reboots. I just want to know if this is legitimate, or if the reboots should ultimately be attributed to an android or handset manufacturer problem.
An app can certainly deliberately reboot the phone, depending on the permissions for the app in question and whether it has root privileges.

An app can also force you phone to reboot if it bombs out and takes Android down with it, particularly apps that interact with phone functionality (volume adjustors, automatic task killers - which are a bad idea in general, location aware apps, etc)  If an app has the right permissions this can happen quite easily.

So to sum up, yes, it's quite possible that an app could be causing the system to die and your phone to reboot.  If you want to troubleshoot, you can use Titanium Backup to freeze one app at a time to see if that alleviates the issue, or you can just uninstall and then reinstall later on.