Spam volume continues to grow. By my measurements, mail relays are now seeing 60-70% spam by volume, up from about 35% in 2001.
What operators are doing
SpamAssassin has matured into the de facto open-source default. The Bayesian filtering produces high accuracy with manageable false-positive rates.
Coordinated blacklists have proliferated. Most operators subscribe to several; the cumulative blocking is substantial.
Sender authentication proposals are accelerating. Multiple competing standards (SPF, Sender-ID, DomainKeys) are in development. None has emerged as dominant.
What this teaches
The spam problem is structural; the responses are partial; the trend continues unfavourably for operators. The structural answers — sender authentication, reputation systems — are years from deployment.
More as the year develops.