When I set about developing a layer of seasoning on my unused carbon steel paella pan, I also figured I'd experiment a little with trying to season an aluminum half-sheet pan, that was already discolored and had developed quite the patina. In using both of the pans to roast things in the oven (brussels sprouts, squash, broccoli, etc) over the last several months, both have developed essentially nonstick surfaces, similar to a seasoned cast iron pan. I'm able to use metal utensils with no problems, and scrub thoroughly with no problems.
Are there any downsides to this? I haven't seen it before, but I love using this pan for roasting.