Despite my previous success, I have found a bug in the way that the PM stationary pad handles quotation marks. This is probably some kind of text encoding issue, but I'm not particularly good at finding those, so I ask Kirk's help here...
If I type this string into the stationary pad directly (keywords field):
Barack Obama 08 "Town Hall Meeting" Austin Texas Democrats
After upload to flickr (via default web upload interface in Safari), the same string has mutated to:
"Barack Obama 08 Town" Hall "Meeting Austin Texas Democrats"
In other words, it has added an extra set of quotation marks which I did not type. For flickr purposes this is obviously bad.
During my previous success, I was uploading from a Leopard startup volume, did my PM tagging in Leopard, and used a third-party standalone uploader. This time, when the issue occurs, I am in Tiger and using Flickr's most basic default web interface.
An additional factor during my success was that I typed my keywords into BBEdit first, then pasted into PM. Users of BBEdit will know that it sometimes has the effect of magically cleansing invisible text encoding problems. However, in this case adding that step to my Tiger flow did not help. My text encoding in Tiger is set to Mac Roman, which I assume is the default, because it's not something I would ever touch.