I posted this on the JIRA a while ago now but no-one seems to have noticed it at all. So yes, this is pretty much a blatant plug, but also I'm interested to know if anyone but me is actually interested in such a feature!
The JIRA issue can be found at this link:
http://jira.secondlife.com/browse/SVC-302
The basic summary is that llGetInventoryUpdateTime() is an LSL function that when supplied a name or key for an inventory item, will tell you when it was last-updated. This allows us to very quickly find new/updated items and avoids having to re-process an object's entire inventory, or maintain internal lists keeping track of update times.
It also opens up an even faster methods of finding updated/outdated inventory, see the sub-task for details of that.
It might not even be all that hard to implement depending how things are organised to store inventory at the moment. My thinking is that it must be some kind of ordered list by name, since a lot of LSL look-ups use the name of an inventory item, and returns items in alphabetic order, a sorted list would make this faster so it makes sense. All that times would require is an extra field to that list so that beside each entry is a timestamp of when it was added/updated.