There is none.

We still have v0.4 revision 1.2 + some ideas for various extension/suggestions, but AFAIK no new established specification and no finished documentaion. If you are interested in the so called v0.6 handshaking and extensions/suggestions like supernodes, query routing, message caches, file hash and metadata... visit and search through:
http://groups.yahoo.com/group/the_gdf/files/
http://www.limewire.com/index.jsp/tech_papers
http://groups.yahoo.com/group/the_gdf/ <- query the archive search

PS: v0.4
PPS: See also this older thread about v0.6