Surely you could implement this as a client side feature, saving the 'viewed questions' as in html5 storage or in a series of cookies?
Obviously this wouldn't be portable across machines and would have some technical complexities (binding views to particular user accounts say).
...but in general, I find that the only time I'm interested in a previously viewed question is during the same session, when I've accidentally closed a window with a bindingly awesome solution to exactly the problem I'm having ...and then, when I search for it, I can't find it again.
Even if this was only a per-session feature, it'd be truely awesome.
(also, if you want to save your session's views, you could totally have an export-as-bookmarklet option).
I realize that your browser should be able to generate this information for you, but both firefox and chrome get so confused by the fact that I visit stackoverflow so much that they list dozens of irrelevant urls that isn't any more helpful than just searching.
The feature I really want is: Where the heck is that question I was just looking at 2 minutes ago?
Edit: I notice the original question specifically said they wanted it to persist across different machines. Perhaps this isn't the best solution for that specifically.