Edit: Licensing has changed so this is no longer true. :(
NServiceBus is still free if you want to download and compile the source, theres no restrictions on it.
The licenses you buy are for commercial support.
If you download/compile it yourself then you can only rely on yourself and the community to solve issues and look for help.
Buying a license you get restrictions on what you can run NServiceBus on, but the commercial support.
(i hope that makes sense)
Edit:
Yes, when Udi Dahan did he training course end of Sydney last year, the issue around licensing was asked during a talk he did afterward, he said NServiceBus is still free and open source, however some companies wont use it because there is no commercial support.
So if you download/compile yourself, its free but you are on your own in terms of support.
If you download it via the website, you will get limited support via the express edition, and buying a commercial license you will get more support, but you are locked down to the servers on the site.
http://sydney.ozalt.net/2010/11/november-meeting-recorded-for-your.html
Video ^ 1 hour 23 minutes in the video is the talk of commercial support.
http://www.philliphaydon.com/2010/12/sydney-alt-net-with-udi-dahan-and-nservicebus-commercial-support/