Matt Cutts has a great post about how Google doesn't trap a user's data.
I think this is part of what makes Google so attractive. If you contrast this post with the ongoing tussle over office document formats (ODS versus Microsoft's Open XML). Part of Microsoft Office's dominance has been their ownership of the default format.
Microsoft isn't the only (or even worst) offender here. I've been recently frustrated by the closed data system of Adobe Elements, and the lack of a document tagging/category system in general. Vista has gotten some heat (I think Jon is correct that there isn't an obvious right/wrong) about how Vista has 'corrupted' some image files by writing tag data into their meta data.
Apple is another offender. My ongoing effort to maintain the ability easily export your Playlist data from iTunes is to compensate for a feature that really should exist in iTunes. At least they made the data relatively easy to access (XML).
I also noticed Matt's post didn't touch on Picasa. Google isn't perfect, and a lot of their existing sharing is for data that has well established formats (Email, Calendar, Docs, etc). Maybe we need a unified approach for Image tagging/categorization.