Confused on scaling memcached & persistant connections

[Mike Lynn]

maybe somone can help me out - but I'm confused on scaling memcached.
Right now I've got 7 web servers each with memcached on them.  All webs
are running apache/php/memcached.   with persistant connections to
memcached.  Each memcached instance avg 2000 connections on avg - how do
i scale that?   Memcached is set at 10k connections max - but is seems
to slow down after 3k connections.  Because the php client lib uses
persistant connections, which is good, it means that each new web server
addes 500 persistant connections to each memcached server - so I've hit
the max???  I don't see a really (good way) to do a truly global cache.
Perhaps a UDP based memcached....?  Thanks for any help in advance..
mike