In 2.1 unsolicited whispers will be throttled in an attempt to reduce spam and advertisements. The actual throttling rate is still being tuned, but it won't affect normal play.
It will however, affect addons which are using whispers for communication.

So... in 2.1 SendAddonMessage() can use the "WHISPER" chat type for targeted communication:
SendAddonMessage("prefix", "message" [,"type"] [,"target"])
The whisper throttling does not apply to addon communication.