I'd consider doing something like dropping the AND by default.
i.e. /photos/london/2011
while still having the OR and NOT and any other operations you need. the idea being that at least due to training from hierarchical filesystems the AND would seem rather implied. As far as considering the duplicate filenames i'd add a prefix to them based on the order they were added to the DB (e.g. first one gets no prefix, second one gets a 2_ or whatever you figure out) that way you won't has as many cases where a file's name changes the moment you add in a new one.
Tagsistant 0.4 doesn't use AND any longer, but that brought the need for a termination operator, which is '=' so far, but probably will be changed for shell comfort.
The 0.2 query mpoint/rock/AND/seattle/ becomes mpoint/tags/rock/seattle/=/ in 0.4
And about the prefix, Tagsistant 0.4 indeed uses a NNN_ prefix to filenames to allow for duplicated names to coexist.
