I'll back you up on this one. This also applies to my iphone version as well. At first I thought it was a security feature that signed you out when you hadn't been using the app for a while. Now, I've come to realize it never remembers the login info once you've exited the app, closed it and then try to resign in.
We have been able to replicate this issue with extensive training. We are currently working on sending out a fix for this problem. We are expecting to have this issue fixed with the next update to the SlingPlayer application for the iPad.
Sling Media Moderation Team