I've been in the habit of using .html for all page extensions for a while now, for one simple and rare reason: if the dynamic technology ever changes (you move to php to .Net, etc) your page names don't have to change. This allows people who bookmarked your pages to always be able to use those bookmarks and for your pages to keep their ranks without extensive or complicated redirect scripts.

A secondary reason is that using .html hides the technology you're using from potential hackers to some extent. It will keep the kiddies at bay at any rate, anyone who knew what they were doing would still be able to figure it out.

Just my 2 cents. Most people would never run into either of these problems on their personal websites.