Hi gals!
This issue has been an ongoing one for me ever since I went Mac 13 months ago.
On my PC, I used PSE Organizer and it's fabulous tagging system to organize my stash. (paper, fasteners, stitching, etc.) The closest thing to this system for the MAC was Shoebox. The only thing, in fact. I never loved it but when it wouldn't stop rearranging my file structure, I finally gave up.
I've bought Aperture AND Lightroom only to learn that neither will tag PNG files. Duh! Most digi scrap files are png!
SO, yesterday I stumble on a forum discussion on another site and learned that Bridge CS3 WILL tag PNG files even though I get an error message saying it won't. Even though it's telling me it won't, it DOES! YIPPEE!!! (CS2 CAN'T tag PNG's, BTW)
Here's the silver lining to the bright cloud...Bridge is writing these keywords to the file itself so I can rearrange my files/folders, move them to another drive, or use a different software, and the keywords will 'travel' with the file!
The next chapter to this loooong and improving story is that ACDSee is working on a MAC version of it's File Manager! While this program uses a resident database (rather than writing to the file), it does give the option of writing to the file where possible (PNG included!). It's in Beta now but once I'm done keywording in Bridge, I can use any program I want b/c the keywords aren't program specific!
I'm so excited I may just make a margarita!!!

WaHooooooo!!!