As Zen Studios have explained on their own forum:
Hi Everyone! Yes, Paranormal will be a FREE download for one week only. It's a bit of a 1 year Pinball FX2 anniversary celebration, and we really wanted to say "thanks" for all of the support over the past year.
Once you download the table it is yours FOREVER.
The FREE download period is Oct 26 - Nov 2.
If you have friends who are experiencing Pinball FX2 for the first time, please make sure they download the FREE Pinball FX2 platform from Marketplace when they download Paranormal.
Once again, thank you all for supporting Zen Studios. It's been a great year for us and we are happy to give something back to you, our fans.
Major Nelson has also announced this fact here: http://majornelson.com/2011/10/18/coming-soon-to-the-xbox-live-marketplace-12/
Happy Halloween, everybody!