haven't seen a thread about this yet, so here it is
WoW will be using Lua 5.1 for the expansion.
For the most part, this is a seamless transition, but there is one syntax change that will need to be made in a lot of addons:
for k,v in table do stuff end
for k,v in pairs(table) do stuff end
Also, the main advantage of Lua 5.1 is the incremental garbage collection, and the full functionality of the 5.1 collectgarbage() function is available to the WoW scripting system.