Samsung themselves say the return rate is below 2%. I guess it depends whether you believe Samsung who presumably have the figures but may be biased or some random bunch of Wall Street Analysts who have no figures and may or may not have a hidden agenda.

So far as I know, you can't install an app on an iPhone unless either its a developer's iPhone or your app is in the store. You can install arbitrary apps on Android phones. If you want to write apps that will be of use to your colleagues or the students but not much use to anyone else then you may be best to learn Android programming just so you can distribute them locally without having to submit them to either app store (assuming of course a reasonable proportion of the potential users are using Android). Otherwise, if they're all iPhone users or you want to target both, you might want to stick with writing web applications (or use an environment like Titanium Mobile to develop for both).

To change the user on an Android phone you need to go into application settings and delete the stored data for "Google Mail", "Google Mail Settings" and "Google Apps", Then reboot and the phone should prompt for your login details. No need to restore anything. At least that worked for me.

No, there are other differences but they are pretty obscure. If you use Google AppEngine then if you have a Gmail address you'll find your Gmail name is reserved so that you can use it as an AppEngine site but nobody else can use it. If you have a Googlemail address your userid is still reserved in AppEngine but you can't claim it. Also when AppEngine first came out it wasn't possible to give a googlemail user permission to manage a site: you could permit a gmail user or someone with a non Google email. They fixed that but there have been several cases where googlemail didn't work quite like gmail on newly released Google applications.

Just for interest, anyone using IE who wants to see what that page should look like can view it at http://dean.edwards.name/IE7/compatibility/complex spiral/demo.html. It is the same page, but using Dean Edwards 'IE7' which is a set of behaviours for IE to improve its compatability.