Thanks for posting your experience, and for jumping in. Just so my response isn't mis-understood, Pandora owns and designs their own App. We've hosted it in products from last year (with very little trouble) and this year (also with very limited trouble).
One thing that you've tried that I didn't do was add it in my Apps Manager. I've always launched Netflix and Pandora from the main menu screen, and never logged in using the Account Manager. I wonder if the log-in process is confusing with the non-login process, and if erasing the login information associated with your account if you can just use the regular App button on the main page.
If I get prompted with a code, I leave the screen up and log on to http://pandora.com/samsung and enter my code and associate my account with the code. Once the website says it's set, I go back to the TV and click "ENTER" on "OK" and the information comes up. I also noticed in the Apps settings that the Pandora account is now "locked" and my only option would be to erase the App.
I'm not sure if this will help, but I'd advise just using the Pandora button on the main screen and attempting the login from that.
We recently purchased a new Samsung Internet TV model LN55C650 and after getting it all up and running and connected to our wireless network, started playing around with the Internet@TV. The first app we tried was Pandora since I love the product and have a paid account. The app would not work. After trying it several times both via being logged into an Internet@TV account that was successfully linked to my Pandora account and not being logged in, it simply would not work. So we tried many of the other internet apps and they all worked fine. So back we went to Pandora to figure out what was wrong with the Pandora app. I'm posting our experience to this forum in hopes that it helps other people as one of the other posts (titled "Pandora issues" by Mike-STL on 8/19/10) in this forum finally helped us to resolve the problem (not Pandora's support staff). First of all, the support person I dealt with at Pandora had the audacity to tell me, and this is a quote.. "We don't have other Internet@TV users writing in with problems." The Pandora app problems reported in this forum are proof enough that this statement is far from true. In any case, the problems were related to the device activation code process not working AND not being able to log into Pandora from my Internet@TV account. The support person at Pandora kept telling me that he didn't see any device activation code associated with my Pandora account after trying it multiple times. He had me reset the TV, which we did and that did not work. He had me upgrade the software on our TV, which we did and that did not work. He even updated my account with one of our device activation codes manually (since their web based device activation code process did not seem to be working (even though it indicated that it had completed successfully.) I kept questioning him about why the device activation code process was not working when it appears as though it is and completes with a message that tells you something to the affect that "you can now use Pandora on your TV". I received no acknowledgement or explanation from him on this. After 5 days of unsuccessful troubleshooting with this customer support person at Pandora, we finally looked for similar problems on the web, which resulted in links to posts about Pandora app problems in the Samsun forum. As mentioned earlier, based on the info in the Mike-STL post on 8/19/10 who had the exact same problems, we deleted the Pandora app and then added it back in. We launched the app and received yet another device activation code. We left this screen up on the TV (we did not exit out of it) and went back into Pandora's Samsung device activation registration area and once again, entered the code and the account info. Once again, the process appeared to work just fine. We then exited out of the is activation code screen and then launched Pandora again. It finally worked. BUT.. it will still not log in to the app if you try to launch it after being logged in to your Internet@TV account. Still getting the message ""Cannot login" "Invalid user-id and/or password". When you link your Pandora account info to your Internet@TV account, it will check out on Pandora and come back and say "Registration successful", but it will not log you in via this log in method. Hope this helps anyone out there to avoid this terrible experience with getting this app to work. Maybe the Pandora app has not been fully tested and not ready to be available on the Samsung TVs, because there are obvious problems that are causing customers far more problems then they should be getting after such an expensive TV purchase.