If I'm not mistaken, a number of plugins on various platforms take it upon themselves to display PNGs for browsers (even >=4.x), with poor results (loss of the ability to do partial transparency, and in some situations, they result in broken plugin prompts). I wish PNGs were safe, but I'm still not sure that they meet the lowest common denominator client that OACS has been so happy to support until now.
Of course if more people took the plunge and started using PNGs, there'd be better support out there... if you're not part of the solution, you're part of the problem.
Short version: I'm torn, but gifs are safe, if not entirely legit or quite as capable as pngs.
