Just seen a link on a booking to a new free nights promotion (obviously IHG's answer to Starwood's similar deal), although not all the links seem to work at the moment.
Celebrate Our Biggest Free Nights Offer.* Here’s how it works:
Join Priority Club® Rewards and register for this offer today.
Stay any 2 nights from 4 May – 3 July 2009.
Get 1 FREE night to use from 3 July – 26 December 2009.
You can earn up to 4 free nights.
Use your free nights at one of over 4,000 hotels around the world.
A few things to consider:
Members will earn free nights instead of base and Elite points/miles.
Register online before booking your next stay.
Receive free night notification via email; email address required to register.
On this basis, I haven't quite worked out whether it is worthwhile or not for my future stays.
The links should be on - no doubt these will start working soon:
www.getafreenight.com
www.priorityclub.com/bookfreenights