Eposible,thank you for purposing such an interesting question. Most conventional historians say that Coptics was divided up into regional dialects like Sahidic,Akhimic,Fayiumic,and Boharic. Of the various dialects of Coptic,Sahidic is probally the most pure in vocabulary. Sahidic in Upper Egypt was spoken untill about the 16th century or perhaps even later. The other Coptic dialects are heavily inflused with Greek. Know what people don't relize is that it can be proven over 14,000 words in modern Egyptian colloquial Arabic trace back to ancient Egyptian. Some linuist even feel that Egyptian colloquial Arabic is really a further development of the ancient Egyptian language. Modern Egyptian colloquial Arabic is divided between two dialects:Cairene spoken in the north,and Saidi spoken from Beni Suef to Aswan.
I am not a linguist,but I think that Dr. Alsaadawi can perhaps answer you question more directly about the ancient Egyptian language being extinct.
I hope he is still here so he can answer the questions.
Here are some further book references if you can track them down at your local book store. See the following
1.Baiume Kandil, "The State Of Culture In Egypt
2.http://www.aucpress.com/cgi-aucpress/auc99/pager.cgi?catno=708_6
From Pharaoh's Lips :
Ancient Egyptian Language in the Arabic of Today
AHMAD ABDEL-HAMID YOUSSEF Introduction by Fayza Haikal
[This message has been edited by ausar (edited 28 January 2004).]