To elaborate, for anyone who hasn't used Photoshop (in several years): you can have layer groups which act as folders containing several related layers, so when you're browsing layers, it isn't just a mess of 500 some-odd rows.
Sounds like what's needed is a way to extract a PSD into a component file structure, and combine that structure back into a PSD. Without knowing anything about PSDs, I assume it would contain lists of layers with links to resources such as images or whatever is required to approximate the desired view. Something like that as a plugin or hooked to run before and after commits would be really useful in this case.
Depending on how hard the PSD format is to grok (purposefully or otherwise), this may be nigh impossible, but it also seems like it would be a great exchange format (which may be a another reason Adobe wouldn't want it). Then again, maybe there's already something like this...?