There has been some slightly misleading press on Hotmail's use of IronPort to provide a whitelist for junk mail filtering. Jeff Patridge posted a conversation with Reeves Little of the Hotmail team where a few things were clarified. The white list is just one of many tools used to combat junk mail. I've been amazed at how little junk mail I get on my hotmail account as a MSN subscriber. They've done an awesome job on their junk mail filters. I used to just dozens of junk mail messages a day on that account. In fact, all of that junk mail used to be a good way to test out Entourage with weird malformed email :-)
Like me, Reeves used to work for MacBU. He was the test lead on Outlook Express and Entourage.