chardev.org seems to be headed that way, still beta and only working with Firefox. Don't know if the guy is still working on it, though.
Apart from that, wowhead.com stated in their boards they are considering doing something like that as well. If they were to actually do that, I'd wait for them to finish before starting my own thing.
And apart from that, I have started on something like that as well before somebody pointed me towards chardev.org. I have since stopped working on it, seeing how chardev was a couple of steps ahead of me already. Not seeing much progress over there atm though, so I might pick it up again.
Regarding the database:
The Armory is your friend. Parseable XML-output is nice to have, though it does have some inconsistencies (sp?) like seperating between attributes (str/agi/etc) and effects (+heal), but still it's easy. I have a .php that parses the results before handing them over to javascript, since I didn't feel like doing searches/comparisons and the like in JS.
The Basics are already working, though it's still ugly (getting stuff to work > design). At the moment I have a working search without reloading the page, (basic AJAX-stuff), though only by name. Should not be much of a problem to add a slot-selection or anything else the Armory can do itself. You can add/remove/replace items in the various slots, while always keeping track of the total stats for your character.
Sockets and Setbonuses are handed over to the JS, though not parsed at the moment. But that shouldn't be that much of a problem as well.
The main thing I'm missing so far is an implementation of a character-planner á la
wowhead's talent calculator, so you can display correct values for, let's say, +heal (=heal+% of int if specced in Holy and the like).
So much for random blabbering about what I know, I'd love to see something like that working again as well.