Try this.
Make all your changes, then set your iFrame code, then save (or publish). Do not go to Visual Editor before you save.
My iFrame codes function properly until you return to edit the page. If you make any changes anywhere on the page, or got to Visual Editor before you save, WordPress 2.5.1 eats the iFrame code. This is different that 2.5.0, which scrambled the code, rendering it useless.
You have to do it all over again if you ever have to re-edit that page.