We’re getting ready for a SCAMPI-A and we use an “agile” method for allocating work to an iteration. In my opinion allocating an iteration backlog based on resources, schedule, etc… from the main product backlog fulfills PP SP 3.2. I wondering if you agree or if I’m missing something?
Nope, you're not missing anything :)
PP.SP3.2 refers to the iterative process that occurs when a plan is reviewed, compared against available resources, and (potentially) revved to match the two things up. This happens on both Waterfall and Agile projects.
In some ways an iterative and incremental ("Agile") approach has the potential for doing this better than in a waterfall world. Since it is re-evaluated against the product backlog before it is place in the iteration (potentially every n-days), and it is done every time, this is is happening repeatably throughout the course of the project.
There is one difference between the two "camps" though. In an Agile world (ala Scrum, for instance) it's the scope (features or stories to be delivered) that are renegotiated, where in a waterfall we usually go back and ask for more money, time, or people.
So, as long as you can demonstrate that you are indeed behaving this way, and you're doing so with integrity, you should have no problem with this practice (PP.SP3.2).