Vadim Zeitlin had these same concerns, which is why he wrote yahoo2mbox (http://www.tt-solutions.com/en/products/yahoo2mbox). This hack retrieves all the messages from a mailing list archive at Yahoo! Groups and saves them to a local file in mbox format. Plenty of options make this handy to have when you're trying to transfer information from Yahoo! Groups. You'll need Perl and several additional modules to run this code, including Getopt::Long, HTML::Entities, HTML::HeadParser, HTML::TokeParser, and LWP::UserAgent.