I've been working on digital supply organization for awhile now, and after studying Peter Krogh's Digital Asset Management, the first thing he teaches is that with any software you spend time organizing things in, the first thing to consider is that if the software you want to use were gone tomorrow, how much of your tagging would you lose?
I was so excited to see that Bridge tagged PNG files but I decided to test it further - so I tagged a file and moved it - the file retained the keyword
Next I decided to save it as is - no changes. Poof the keyword tag was gone.
So I add it again in Bridge and then open the file in Photoshop and choose File > Info and no keyword.
I also opened it up in the new PSE9 Adobe Organizer - no keyword.
This is because PNG files do not hold metadata - there is no standard place for them to do so. The Library of Congress has abandoned the PNG file format as an archive format because you cannot reliably add metadata to it.
So for a visual example, the PNG file format is like a house plan. JPEG, TIF, PSD house plans all have similar closets where metadata is stored so different programs know where to look for them. However, the PNG format doesn't have any closets.
So while it may look like your PNG supplies are organized and tagged - the reality is when your software is upgraded, the file changes, etc. You can lose all that work.
The only reliable way to do this is to change your file format to PSD or TIF. The bummer is that the file sizes increase, however, for long term organization I have found it to be worth it.
I also use Lightroom to organize all my supplies. Love it! I even changed my file format from PSD to TIF midstream - having to reimport completely new files and all my keywords stayed!! So I am excited to have a system that actually works long term no matter what software program I use to organize my stash!
hth!