This is my opinion only, not confirmed specifically by Adobe. I base my statement on this blog.
In order to qualify for upgrade pricing when CS6 releases, customers will need to be on the latest version of our software (either CS5 or CS5.5 editions). If our customers are not yet on those versions, we’re offering a 20% discount through December 31, 2011 which will qualify them for upgrade pricing when we release CS6.
So if you are on the fence about upgrading I would say do it now and save $500. I could be wrong, they could stop people from skipping one version of the CS upgrades on all other products, but still give pagemaker users a break after skipping 7-8 (?) versions.
Having said that I'll explain why I'm not upgrading. I have CS5 on my mac, I also have a copy of Pagemaker 6.5 on my PC. I had been considering upgrading my PC copy, just to stay in the upgrade stream, figuring they wouldn't keep the Pagemaker upgrade forever (my older PC copies of Illustrator and Photoshop no longer qualify for upgrade prices). Now I would have to upgrade the PC copy every time to stay qualified, and it is not worth it.