Duncan Riley at TechCrunch writes:

Akismet is a service that relies on the failure of the WordPress code to be able to natively deal with comment spam. The service is free for personal use and a paid service for everyone else. As the co-founder and essentially the head of the WordPress open source movement, Mullenweg leads the initiatives by WordPress to combat comment spam. On the other hand as the head of Automattic he runs a company that profits from those very failings. The question then becomes: can one profit from the failings of an open source product whilst still leading that very code’s development?

Wrong. The problem Akismet addresses is a general problem, not one specific to WordPress and it would be less effective for every blog hosting services to run their own comment filtering service. IMHO, fees Akismet charge are well-earned and kudos Akismet receive, for offering free comment filtering service to non-commercial bloggers, is well-deserved.

As to whether there is an acceptable line for advertising and conflicts of interest, my answer is yes but does it really matter when the line can be redrawn?