Mobile apps: Where to store web service login details

[Keith Clarke]

Hi Folks,
I'm dipping my to into mobile development and looking for advice on where best to store web service usernames and passwords that are accessed from the device (with security and persistence between session). 

The use case is a simple iOS/Android app that would access my revServer.

I can't find any reference to this in the docs - am I missing the magic search keyword?
Best,
Keith..