The long wait is over. Bioware has announced that Return to Ostagar is now available for the Xbox 360.
The PC version will follow later today, with the PS3 version being released "later".
UPDATE: Chris Priestly has announced that Return to Ostagar has been removed from XBox live. He says:
"This morning a title update on the Xbox 360 for Dragon Age: Origins was made available in preparation for the release of Return to Ostagar. This title update introduced a previously undetected issue that causes specialization classes to not work correctly in Dragon Age: Origins. We are removing Return to Ostagar from Xbox Live until the issue is resolved and are recommending that you do not download the title update. If you have already performed the update,…Read more >