Steve Dispensa and Marsh Ray have published a paper describing a weakness in the TLS negotiation process. This is the same attack discussed on the IETF TLS list.

From the whitepaper

"Transport Layer Security (TLS, RFC 5246 and previous, including SSL v3 and previous) is subject to a number of serious man-in-the-middle (MITM) attacks related to renegotiation. In general, these problems allow an MITM to inject an arbitrary amount of chosen plaintext into the beginning of the application protocol stream, leading to a variety of abuse possibilities. In particular, practical attacks against HTTPS client certificate authentication have been demonstrated against recent versions of both Microsoft IIS and Apache httpd on a variety of platforms and in conjunction with a variety of client applications. Cases not involving client certificates have been demonstrated as well. Although this research has focused on the implications specifically for HTTP as the application protocol, the research is ongoing and many of these attacks are expected to generalize well to other protocols layered on TLS.

There are three general attacks against HTTPS discussed here, each with slightly different characteristics, all of which yield the same result: the attacker is able to execute an HTTP transaction of his choice, authenticated by a legitimate user (the victim of the MITM attack). Some attacks result in the attacker-supplied request generating a response document which is then presented to the client without any certificate warning or other indication to the user. Other techniques allow the attacker to forward or re-purpose client certificate authentication credentials."

The full document in pdf format: Renegotiating_TLS.pdf
Some helpful protocol diagrams: Renegotiating_TLS_pd.pdf
Packet captures: renegotiating_tls_20091104_pub.zip