Using phpsmug I have a working cache but am having problems coming up with a way to clear it if the album is updated, rather than wait for it to expire. I was going to use the ["LastUpdated"]=> value from album info but this only seems to update if you add or delete content. If I change a caption or reorder the images it stays the same. Is there another value I can key in on to do this? Or perhaps a better technique?
I've been exactly there. What I've learned is that there is a specific set of actions that invoke "LastUpdated" pretty much what I've tried to explain here
Coupled with the basic caching feature of phpSmug, I'd say you have your work cut out for you. The author of phpSmug is considering expanding it somewhat (like, pull a fresh copy to the cache in the background when it's about to expire) maybe he can comment on this himself, as he is known to be lurking this excange from time to time...
Overall, the inability to control what is considered an update in the LastUpdated attribute and the current caching scheme of phpSmug led me to scrap my plans to set up some intelligent sync. You might fare better if you take the plunge with considerable skill, best of luck mate!