Realistically, feedback is feedback. When you take away the way it was delivered, you're left with the simple message. No matter when your former company asked you for the feedback, they still asked the question. Besides, giving feedback with tact can still be as pleasurable and it may show of qualities in you that you may have not previously demonstrated, which could ultimately leave the door open for reemployment...if you had to. You could also help out now former coworkers improve their current situation.
The important part of all of this, and where some people go wrong, is the way the feedback was delivered. Often when people are in these kinds of situations, especially if you're on the getting canned side of things, their emotions are running high. This will sometimes mean that you're running in more of a 'primal' mode rather than thinking about your message. As a result, it can look like a impassioned rant or a blubbering pile of incoherent blabbering (on the extremes). If providing feedback, it's important to place this kind of feedback in an actionable, realistic way. Keep emotion out of the situation.
When responding, try to answer in capsules of information presented in a very factual, respectful manner and including details along the way. For example, "I find that the relationship I had with Peggy Sue was great in many ways. She always greeted me. Though, there were some challenges that ultimately resulted in my decision to terminate our employment relationship. I found that her style was a little too hands on and I felt like she didn't trust me to run with a project to the finish line. For example, recently we were working on using genetic cloning to make a white horse that also grew a unicorn horn. I almost had the base sequences aligned and only needed a few more days to make it perfect. Peggy Sue surprised me with a check up regarding this project and was in the impression that I had finished my plot and decided that she would now take over the project and assigned me to start figuring out how to make pigs fly. This is one example of this type of behavior, I can provide more if needed."
Another critical part of delivering the how is making sure your tone of voice remains neutral, even slightly positive and forward thinking. Since this is text based, I'll try to explain my meaning... Think about a time when you've gone out for dinner and the server you've had is simply amazing. The kind of experience you walk away from saying 'yeah, they were good!' What did their tone of voice sound like? I'm not really saying you have to sound like a sultry temptress (damn you, hooters girls!) or that really flaming gay dude at the gap....but they were into their job and had passion in their voice enough to say that they really cared about the situation, but not enough to get creepy about it.
PS: Huzzah for first registered /. post! =d