http://nvd.nist.gov/nvd.cfm?cvename=CVE-2007-1103 "Tor does not verify a node's uptime and bandwidth advertisements, which allows remote attackers who operate a low resource node to make false claims of greater resources, which places the node into use for many circuits and compromises the anonymity of traffic sources and destinations." All <= 0.1.1.26 versions reportedly affected. Upstream statement: http://blogs.law.harvard.edu/anonymous/2007/02/26/the-rumors-of-our-demise/
Closing: I believe this is a design issue and relatively well documented: "Feb 25 16:16:02.628 [notice] Tor v0.1.1.xx. This is experimental software. Do not rely on it for strong anonymity."