Thanks stef, let me answer some of those...
1) Zenphoto uses Object-orientation where it makes sense. The representations of Gallery, Image, and Album are objects, but most of the things that deal with their control are functional. There's no reason to use objects for everything when it makes more sense for parts to be functional, like the template functions, or common utility functions. So no, I'm not "moving the code to more object oriented perception" -- it was designed exactly that way from the beginning, and it's not becoming any more or less so (until perhaps version 2.0 with a revised model and more classes of objects, but even then objects will only be used where they make sense).
2) The main use of an API doc will be for the template functions -- if I comment them enough, the PHPdoc output will be very useful, as people will be able to see exactly what each function does for their themes. Also, the class documentation will be useful for theme developers, since you can still skip the theme function layer and use the classes directly for advanced operations. Plus they will be useful for people wanting to help develop (and I want people to do that), submit patches, and understand the codebase better. So yes! Very useful.
3) You're doing great so far, if you could keep updating the PHPdoc when I add more comments that would be good, I'll let you know when I do, or you can keep track of SVN. Also, I'll try to integrate EXIF with the database so it's easier to work on with Zenphoto. Next step, just try to understand the code and begin to see what could be improved, what could be faster, etc.
You also mentioned speed problems in your e-mail -- Zenphoto is very fast algorithmically, but currently works off the filesystem, which could be a major speed bottleneck. In version 2, I'm planning on making it *based* off the database, with periodic and strategic disk checks to have the same kind of "automatic albums from folders" feel to it, but faster.