The OpenStreetMap API is now in Read Only mode while the upgrade/migration to 0.6 is undertaken. The API will be in this mode (or at times totally unavailable) until the changes are complete, which at the moment is expected to be some time on Monday April 20th.
During the upgrade OSM contributors will not be able to save their editing to the OSM database and the online editor, potlatch, will be disabled.
Once the upgrade is complete be sure to also upgrade your OSM editing software to ensure compatibility with API 0.6. Potlatch will be updated as part of the migration process.
For more information on the upgrade see here