Yep. Starting with build 2.2.1 (2.2.2 is the current build on the Nessus DL site), Nessus now requires you to activate the software. Apparently they have been getting ripped off by a number of folks who are profiting from the GNU product. The good news is that registration is free.

Go to:
http://www.nessus.org/download/

Once you register, you will receive your activation code via e-mail. When Nessus finishes installing, it will prompt you for the code. Once you enter it, you can use Nessus as you always have in the past.

Anyway, FWIW.

--TH13