Yes, a perpetual license upgrade is still possible if you discontinue your Cloud membership but your CS5 Master Suite license can only be used to upgrade to CS6.
Cloud membership is linked to your Adobe ID (one membership per ID). Perpetual (traditional) software licenses are linked to serial numbers.
Under the new one-version-back policy, when CS7 ships (perhaps 2014?) only CS6 perpetual license owners will be eligible to upgrade. Everyone else pays full price.
In effect, you can use your Cloud membership as a paid-for extended Trial period while CS6 is current to see if the Cloud suits you. If you decide you don't want the Cloud, you must upgrade to CS6 before CS7 ships. If you don't you either stay with CS5, or use the Cloud in future or buy CS7 (or CS8 or CS9) at full price.
That's the position I'm in.