(warning: this is pure speculation, someone correct me if I'm wrong!)
omegadude: I think in order to use FEATURES from newer expansions, they would have to upgrade to a newer game client, which, considering how much custom work has been done on the server-end, might be a pretty ugly thing to do, so I wouldn't expect that to happen anytime soon. And I'm pretty sure they would have to upgrade to an "official" version of the client (such as Titanium or some specific patch date) -- I don't think they can legally patch the client themselves.
As far as content, I'm guessing it might be possible to use zone/graphics data (items & monster models, etc) from recent expansions, just without the extended features that those expansions brought unless the client was upgraded.
But, I think the real question is, why WOULD they? Barely any content is used past Classic and Velious, and I think the next "expansion" they are leaning towards for pulling data from, is Kunark (at least last I hear, which was quite a while ago). I'm sure it will be plenty long enough before all the content available up to LDoN gets used up, so I wouldn't really hold my breath for anything beyond that...