It seems that Tennable are going to start charging to use Nessus commercially.  The Carnal0wnage blog does a good job of highlighting the pros and cons to this, so I won't repeat those views here.

Maybe now would be a good time for the pentest community to get behind OpenVAS - an open source fork of Nessus.  It's a very promising looking project, but still needs support to come up to the current Nessus standard.

If anyone knows of another good free VA tool, drop me a mail to pentestmonkey at pentestmonkey dot net and I'll post and update here.