I'm working with a client that's adopting agile. One thing they've done is build their own agile PM tool. It's not bad, but not as good as Rally or VersionOne.
One thing the tool allows is to assign user stories to the "front burner" or "back burner" with the idea that if it's on the back burner, you get to it if you have time...but you still assign it to an iteration.
From an agile perspective, I think this is a bad idea. Iteration planning is about getting the team to commit to the work of the iteration. A story is either in or out. This tool allows you to put a story somewhere in between. It's assigned to the iteration, but it doesn't show up in your story point count or the standard view of the story board. The product owner may think it's in, but the team can easily forget about, leading to disappointment when the iteration is over.
It's important to have transparency across the project. The product owner and team need to be on the same page without any ambiguity about what may be in or out of scope for the iteration.