Blizzard's will be receiving major upgrades soon, due to Diablo III's pending release. According to Frank Pearce of Blizzard, the development of Diablo III has revealed to them that it will put a toll onto the servers, enough of a toll to warrant changes to the system.

Whether that involves simply adding more hardware to the system or redesigning software components isn't mentioned, but I imagine it would be both. It was mentioned that a group has been specifically working on functionality, for both Diablo III and StarCraft II. It also isn't mentioned what it is about Diablo III that makes it so much harder on the service.

The updates to are due along with the release of StarCraft II, which will precede Diablo III.