Some buggy clients (e.g. some versions of Flash-based file uploaders) do not correctly submit the session cookie, so visit tracking and identity handling will fail for these request, since the each request will get a new session ID.
The visit framework currently only looks for the session ID in the request cookies. It should support getting the session ID from the request params as an alternative, so you can work around the buggy clients by transferring the session ID with the request params.
I'm proposing to change turbogears.visit.api.VisitFilter.before_main to retrieve the session key from the request according to a new configuration settings that specifies the methods to use and the order in which to try them. For example (in analogy to the identity.source setting):
visit.source = 'cookie,form'
For background information, see this mailing list thread:
http://groups.google.com/group/turbogears/browse_frm/thread/f5f1e7331685f95d