The last patch specifically addressed the memory leak issues, or at least some of them. Personally I saw a noticable decline in performance from patch to patch using Windows XP 32bit with 2gigs of RAM. It got to the point where I had to keep the task manager open on my second monitor so that I could manually end the AOC process whenever the mini-map greyed out and my system started to chug. This was a fairly regular occurance, resulting in restarting the game approximately every half an hour.
Prior to the last patch I had upgraded to 4gigs of RAM and Windows Vista 64bit and now the only issue I have with the game is the occasional server lag spikes.
Obviously this isn't the preferred fix to the memory leak issue, but if you have the disposable income to run out and upgrade your machine for no other reason than to play AOC, it seems to help. Anyhow, I haven't had any problems with Vista and the game actually seems to run better with my 8800 GTX card, both in appearance and increased framerate. (I also switched from shader 3.0 to 2.X permanently to rid the problems in some of the instances where my framerate would die due to some particle effects or textures, I haven't switched this back under Vista so I cannot comment on whether these changes have any effect on that particular problem).
Anyhow, to specifically answer your question. No, I don't think they have fixed the memory leak issues but it appears it is towards the top of their priority list.
Hope this helps..