asterisk users - Aug 2007 - Client-negotiated Codec Instead of Transcoding?

Is there a way for voice media clients (like SIP phones and POTS/PSTN
phones) that connect their call legs to Asterisk to negotiate a common
codec that they both use at their end, so Asterisk doesn't have to
transcode? Asterisk would know which codecs each client can use, and
which each prefers, then find the one they each have in common so the
fewest legs need Asterisk to transcode to their "odd man out" codec. 
-- 

(C) Matthew Rubenstein