In June 2010 the Bitcoin reference software version 0.2.10 introduced a change to the protocol: the 'version' messages exchanged by nodes at connection time would have a new format that included checksum values to detect corruption by broken networks.
All other messages already carry a checksum (for connections between nodes 0.2.9 and later) but the version messages themselves could not be changed in a compatible way, so this change was delayed and did not take effect until midnight UTC on Feb 20th 2012 to leave users time to upgrade.
The developers of the Bitcoin reference software are unable to find any evidence of any nodes still running software prior to 0.2.10 on the network. If any nodes with software this old do still exist, they will no longer be able to connect to newer nodes.
This switchover has been tested and no significant disruption is expected and none has been observed so far. Nodes with incorrect clocks may have a difficult time making new connections for a brief period around the switchover time.
Please report any new connectivity issues to the #bitcoin-dev channel on Freenode IRC.
If there are unexpected difficulties this page will be updated with relevant information.