Thumbs Up/Thumbs Down Rating System

Ok, we've all seen sites where there is a comment/review/etc where there is a rating, either a count of thumbs up/thumbs down, or a rating from 1-5 with starts.

I'm going for the one with a count of clicks on the "thumbs up" and "thumbs down" icon.

Now the code to do the basics of this is pretty simple, however, while I have idea's to keep it from getting "spammed". There is the option of making sure you are logged in, but for now want to see how it goes for the general public.

Now I know if someone really wanted to, they could find a way to still spam it, but here are the general guidelines I'm thinking of, and would like other people's thoughts on ways to prevent it without using something as bad as captcha...

1. Sessions will be used to to keep a "last visited page" along with a timestamp of that page visit. You have a certain amount of time after hitting a page to click the thumbs, otherwise they don't count.

3. For each comment, it records the last IP, Session ID, and timestamp of the last vote. Never can the same session vote more than once on the same comment ID. I may expand this to move the voting history to it's own table that auto deletes records after a week.

4. If it is the same IP as the last vote, but different session, you can do two votes, then have a 10 minute timeout. (remember, in an office setting, many people may have the same IP address)

5. If there are too many votes on an item within a given time frame, say more than 4 a minute, it disables it for 30 minutes. It will log these as well, so if we see that hey the site is really popular, and it's happening often and logs look like valid votes, then can up it.

Other than the actual logging into accounts and recording all votes, can you think of any other steps you would take?

Note, that for my case, this is just for commenting on products, not throat competition for votes or anything.

Thanks.

-Greg