OpenSSH Challenge-Response Buffer Overflow Vulnerabilities
Joe Testa <firstname.lastname@example.org> has provided information on how a server segmentation fault may be produced with the use of a modified, malicious SSH client. Details are available in the referenced Bugtraq message post.
Christophe Devine <email@example.com> has published a proof-of-concept exploit (as a patch to the OpenSSH client).
GOBBLES has also released proof-of-concept code.
UPDATE: Core Security Technologies has developed a working commercial exploit for its CORE IMPACT product. This exploit is not otherwise publicly available or known to be circulating in the wild.