Browser()I eventually found that my system (currently Ubuntu 8.04) had the package "python-mechanize" 0.1.7b, which masks the one provided by Zope 2.10 (0.1.2b). Unfortunatly there has been API changes between 0.1.2b and 0.1.7b... (see classes UserAgent/UserAgentBase)
The solution is to tell buildout to get mechanize egg 0.1.2b: this way it will mask the system library.
versions = versions
mechanize = 0.1.2b