I'm just curious. For those of you who has written a plugin to publish to sites like Zenfolio, Picasa and other sites that are able to display keywords and other metadata - do the plugin send the metadata along with the photo or is it common practice for the server side to extract the data themselves?
It depends on the service API - majority accept tags along with photos, and also allow to modify metadata separatelly.
However, some services also collect metadata directly from photos you're posting, and this is not always good for the end users.
For example, if user wants to publish photo wihtout any tags, and plugin does not really send them to the service whie publishing - service may itself collect tag information from the picture and present it. This will be agains the user's desire. In this case it's advisable to use minimum settings while exporting/publishing pictures (for example: Metadata -> Copiright only)