Background (Mobile) Web Authentication

I’m building an iPhone app that, in part, allows the user to log in to, pull data, and post data back to a Buddypress site. I’m considering approaching this problem by using a series of UIWebViews (basically multiple instances of a mobile browser window) to display the mobile-themed version of the site.

Ideally, I want the user to be able to, upon first-launch of the app, input their username and password, and then never have to do it again (functionally similar to tons of other social apps out there, like Facebook, Twitter, etc.)

Here’s my question – obviously it is easy to store the user’s login credentials, but how do I, in the background, establish an authenticated and persistant session with those credentials each time the app opens, so for the user, they are never presented with a login screen again, and can just use the app like normal? Keep in mind that this is a session in the mobile-site, not just XML I’m looking to pull via XML-RPC.

Does anybody have any suggestions?