check out my first two posts in this thread about logging in on iPad/iPhone. I realize you are not having the same issue, but the cause is the same: blocked cookies.

http://texags.com/forums/69/topics/2509867

...also, after you've checked on the two items from that other post, make sure you have the permanent website mode set to Mobile in your account settings (this is a per-device setting)

To do this, on your iPad, switch to the desktop mode, go to your account settings, select Mobile under Website Mode and then switch back to the mobile version (at bottom of page).