Replying as both a curator/archivist and as a long time professional photographer, basically you are in the middle of a whole can of worms... you've got photographer's copyright, you've potentially got the copyright of some of the objects photographed (and if they are protected by copyright or not), you have exceptions to copyright, you have property rights etc etc.
 
Some aspects of copyright law fall in your favour, others don't. Also, your current photography policy has an impact on it all.
 
This is complicated by - as someone pointed out - web 2.0 and what the reality of the situation is, as opposed to the legality (in good part, while many aspects of copyright law do still apply to what is happening on the internet, they are woefully inadequate and antiquated in terms of practical application).
 
To actually take any kind of legal action on the situation you mention would most probably require deep pockets, among other things. What's most useful would be to deal with the practicalities of the situation - and as you mention, focus on your current policies. Decide what it is you are happy to permit and then try and craft things to enable that.
 
As for Flickr, it's a community (among many other things - look at the archives that are placing whole photo collections there on-line). What this Flickr member is essentially doing is in part the digital equivalent of the old after dinner slideshow of "our trip the national gallery" (and with a digital camera, you aren't paying for film - so heck - why not take close-ups of the labels as well - saves you trying to remember them) - it's just that anyone can take a look if they want.
 
But, this is just the start - take a look at this video from the TED talks about tagging, Photosynth and Seadragon. 
 
http://www.ted.com/index.php/talks/view/id/129
 
Pretty soon, anyone who wants will be able to draw together all the pictures of your museum that are on the web and build a composite of the any exhibit they want - all it will take is for every visitor to take just one picture and then post it some where on line.
 
From my viewpoint, in most cases (aside from someone blatantly trying to rip something off, appropriate it and/or make a bundle off it where that is explicitly forbidden by current copyright) it seems best to accept what's happening, embrace it and try to work with it. It's too easy to end up like King Canute otherwise.
 
tim a

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