I am pondering the process of migrating this system to WordPress and am using this post as a place to maintain links to possible solutions. I welcome feedback on migration from any reader.
Paid service — http://worth2read.org/blog/2009/01/31/import-from-b2evolution246-to-wordpress-27/
Seemingly good option courtesy of WordPress forums — http://wordpress.org/support/topic/215389
A bit more thorough write up of the process at the WP forums — http://www.seo-strategies.org/2009/10/migrating-b2evolution-to-wordpress/