WordPress 2.5.1 seems to have an issue with iframe as code disappears if you open or edit the post/page in Visual Mode and save or publish it.
Until the bug is fixed, avoid opening the post or page (with iFrame code) in Visual mode and open it in HTML mode. If the iFrame code is disappeared put it back and save the page again in HTML mode.
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If you’re talking about a WordPress.com (ie WordPress-hosted) blog, iFrame code is stripped automatically because of security issues.
I lose a lot of code in posts when someone edits the post in visual mode. Fortunately the code is in the older revisions and I can copy paste the code again in HTML mode. But this routine is getting ridiculous. WordPress needs to fix this.