The bug is now fixed, check out your previously bugged decks in the deck editor :). Original message follows.
Re-posting official statement form Wizards:
We are aware of a launch issue that can sometimes occur with the XBLA version of Duels of the Planeswalkers 2012 that we are in the process of correcting and wanted to help you avoid.
If the user unlocks a deck with a deck key and then makes any changes to the current (moving at least one card from the full deck to the sideboard, including moving the card to the sideboard and then back into the active deck), they MUST "Save Changes" before exiting the Deck Manager to ensure deck corruption does not occur. Whenever the user attempts to exit (pressing 'B') the deck manager after any cards have moved into or out of the deck manager, they are prompted with the message "Warning: You have not saved your changes. 'A' Save and Quit, 'B' Quit without saving." If the user presses 'B' Quit without saving, there is a chance that the users deck will be corrupted.
To avoid this from potentially occurring, we recommend that you always save your deck after making any modifications to it. We will keep you informed of the status of this issue and its resolution as it develops. We are working on an update to fix the bug and hope to have it released shortly.