At present the Lich King requirements are stated but not enforced. The game will load up and run on 10.3.9 and a G4 system, and we still test on those configs. So if you do choose to install it and find that it runs well enough for you, there's nothing being done presently that would prevent that.

Existing Classic and BC players on such systems shouldn't see noticeable change in performance when 3.0.2 release comes out. The bigger issue with Lich King is that the newer areas are more graphically intensive and this is why we telegraphed a change in specs specifically for the expansion content.

The subject of the thread isn't ( IMO ) strictly accurate. You can absolutely continue to play WoW on the machine you have, and possibly even the LK expansion too, that's all up to you.
http://forums.worldofwarcraft.com/thread.html?topicId=10971857617&pageNo=5&sid=1#91