A post to the Diablo II General Discussion messageboard (thanks Flash and James Chiang) offers precise word on when and why to expect a version 1.09 update for Diablo II. Here's the poop:

All realms will be shut down at 9:00 A.M. PDT (UTC -8) on Monday, August 20th for approximately 2 hours. Diablo II v1.09 will be distributed to all the servers during this time.

REMINDER: Diablo II v1.09 will fix the "dual wield" bug. Any characters exploiting this bug when Diablo II v1.09 goes live will lose one of the weapons involved.