IPad 2 with IOS5, Can't set email aliase on iCloud as default

IPad 2 with IOS5, Can't set email aliase on iCloud as default.

I have a new iCloud setup and i setup a aliases to it. On the iPhone5 I can setup the aliases as my default email address but on the IPad2 I have to select it each and every time. How can I make it the default