My upcoming WebTechniques column (not yet in print, so not in the website) talks about "branding" a particular browser with a cookie, and then associating that cookie with a user for a time-limited period of time, with the association maintained server-side so there was no chance of spoofing the server. It's actually quite simple with File::Cache. Took about 40 lines of code. Maybe I'll post it as a snippet.

-- Randal L. Schwartz, Perl hacker