If you’re looking to support iPhone on a SharePoint 2010 site that is configured to use an external identity provider like Ping Federate, ADFS 2.0, or a custom STS, you will likely run into this issue. However, you may notice everything works just fine with iPad. Fortunately, the issue can be reproduced on the desktop using Firefox or Safari by emulating the iPhone user agent and you’ll be able to see what’s going on in Fiddler.
As seen in the above screenshot, you’ll notice an additional redirect to /_layouts/mobile/mblmultilogin.aspx just before hitting the /_trust/default.aspx page. And indeed this is what causes the issue. To correct it, you’ll need to update iPhone Safari section within the Web Application’s compat.browser file and set the “isMobileDevice” property to false.
After making this change, your iPhone users should be able to enjoy the SharePoint experience from their phones. Yeah, that was a bit of sarcasm there….