Can you partition it? Have one portion of the data available to all and sundry, then have another more valuable part hidden behind a captcha'd login? I know th

Can you partition it? Have one portion of the data available to all and sundry, then have another more valuable part hidden behind a captcha'd login? I know this won't prevent anyone else from being able to match your Google ranking on the free content, but it should mean that you can maintain your position as the "default place to go" for it, which should come with attached PageRank goodness. It also means you can institute per-account rate limits, rather than IP-based ones, which might help.