An official post on the Battle.net website, spotted by Kotaku, explains the details of the bans:
As part of our continued effort to ensure a fair and fun online experience for all Battle.net players, we have expanded our efforts to remove cheaters from StarCraft and Diablo II. We have identified and closed over 350,000 StarCraft and Diablo II accounts which were found to be using third-party hacks.Battle.net will see upgrades including advanced cheat detection in time for the releases of sequels StarCraft II and Diablo III.
The Diablo II CD keys associated with the closed Diablo II accounts are now restricted from playing on Battle.net for approximately 30 days. Repeat offenders will have their accounts closed and their CD keys permanently banned from Battle.net.
As a reminder, we reserve the right to close the accounts and ban the CD keys of players who are caught cheating on Battle.net. Cheating ruins the game experience for legitimate players, and we will not tolerate it.