Battle.net Forums outline plans for an upcoming version 1.13 patch for Diablo II:

We’re preparing for a Diablo II ladder reset to coincide with the release of the 1.13 patch that’s currently in development. We don’t have a final release date for the patch yet, but we’re providing early warning as a reset could occur as early as the end of April. We’ll be providing updates with more certain dates once the patch is closer to release.

Please be aware that with a ladder reset all ladder characters are moved to non-ladder. This means that your characters will still be accessible but will no longer have access to ladder-only features and games. In order to play in the new ladder season after the reset you will need to make new ladder characters.

This thread will be updated with more information as it becomes available.