VentureBlog has an interesting bit on spam, claiming that spam is going to give Microsoft control over the entire email server market. The logic is kind of interesting; basically it boils down to using your Exchange server license as a bond against sending spam. If you spam, they yank your license, so owning a valid Exchange server license is an automatic key to spam whitelisting:

However, corporations are already shelling out big bucks for email - specifically for Microsoft Exchange or IBM/Lotus which between them have 75% of the corporate market.

Microsoft could just provide a stamp on each outgoing message (think public key cryptography) identifying that it came from a specific exchange server. This would be verified with Microsoft, which would provide a whitelist of valid exchange servers to every anti-spam company. [VentureBlog]

Three problems with this:

1. Bayesian filtering seems to work really well. My home email filter is over 99% effective right now, blocking roughly 200 messages per day with no false positives.
2. Spammers are already using viruses to generate open relays. How long will it take before office computers are attacked deliberately to use their whitelisted Exchange server for spamming?
3. The liability issues of point 2 will effectively keep Microsoft from blacklisting large customers, even when bushels of spam are pouring out of their servers.

So, in short, I think it's a neat idea, and I wouldn't be surprised if Microsoft tries it, but it isn't going to help. In fact, it'll probably just make corporate PCs even more attractive to spammers.