Sharepoint - SharePoint repeatedly prompting iPad users for credentials

As the iPad is not a domain device, it doesn't have the capability for cached credentials. The most robust solution I've seen on an iPad uses TMG to handle the authentication component. When the user hits the SharePoint site, they're prompted by TMG for user credentials. TMG then securely processes the authentication and provides the handoff to SharePoint. You can also implement a SSO style solution using this same method so that as users move from one web application to another (ie: intranet and mysites) they are not prompted for additional authentication, as it's all funneled through TMG.

This may be a heavier solution than you're looking for, but it is a more robust way of handling that mobile device connectivity end of things.