Let me start with off with saying that software engineering is my favorite thing in the whole world. I started to do thisone day... then I realized that it was completely in veign...
theres really not very many classes... only one thing (entities) which really do much inheritance... and most classes have a shit ton of members... like client has well over 500 methods, and who knows how many member variables.
Basically, the diagram would be crazy messy, and would not make a lot of sense... even if you excluded most of the methods, some of them are related to every other class.
"some day" I want to rework a TON of code to take advantage of a better OO design (and piss off custom server admins (

, but who knows if/when that will ever happen.
I have also looked at making a schema diagram, so people can understand the DB.. that is more reasonable, as long as you dont put the columns in the diagram, because items has like 100+ columns, character has a ton, and so does npc_types.