I've seen this bug mentioned elsewhere online, but the closest reference I found in the bug trac makes it unclear to me if it is planned to be fixed. http://core.trac.wordpress.org/ticket/6807
When writing a post or page and I want to insert something like an Amazon link or Google Calendar with an <iframe> I switch to HTML view to paste in the code. After pasting in the <iframe> if I switch back to Visual view, the <iframe> will often disappear without notice. This also happens with existing posts containing iframes if I'm in Visual mode when I click to edit it.