ERROR: This Page has been modified since you opened it. You must open the page again. Refresh Page. This error was reported on only one site collection. This site collection was in it's own content database, over 100GB. Four web front ends and all reported the same error. Professor Google reported only custom pages and page layouts etc. Our problem occured on items that was out of the box, it occured on viewing lists, adding web parts to pages etc.
Further investigation showed that scheduled maintenance over the weekend did not take place. So the content database was defragmented, not a lot, but it was.
SharePoint SP2 was supposed to do the defragmentation online, which it didn't. After running rebuilding the indexes, the problem was solved. So this error has something to do with the indexes on your content database. I suspect the bigger your content datbase, the more you are likely to get this error if the indexes were not rebuild.