Well...I’ll put my two cents in....
I think one thing everyone should realize is that the support for PNG alpha transparency is still not quite where it needs to be yet. Granted, the support is there for many browsers and with some hacks it can be implemented. But relying on it and designing a site around it might not be such a good thing yet. I always do everything in terms of the "lowest common denominator".
For example, when it comes to screen resolutions, 800x600 is about the lowest common denominator with 1024x768 quickly becoming the norms. Now, this assumes that you’re targetting the widest audience. If you know that the majority is using higher screen resolutions then 1024x768 might be your target resolution.
Another example would be which revision of CSS to use. Obviously, CSS 3 isn’t really an option right now. But you might want some CSS 2 stuff. So developing your style sheets to degrade properly would definitely need to be addressed.
The point is that if you use PNG alpha transparencies, you’ll need to think about how it’ll degrade on browser that don’t support transparencies or PNG’s for that matter. I certainly wouldn’t like to visit a site and see a nasty grey box around an image. I guess it just really depends on how you plan to use them and what your target audience will be.
Just some food for thought.