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I've found it helpful to ensure that somewhere or other in the website there is a straight HTML link to the original document in an iframe, the one loaded by the src attribute. That link can even be in the site map, but that's my only caveat.
And of course, it very useful to have some kind of scripting to help the visitor who arrives at your iframe intended page, but orphaned from its parent. At the very least, make sure there is on-page navigation in any iframed document to allow visitors to access your full site.
Bots have had certain difficulty in that they just see a white box with nothing in it, but will read the url marker we put in the box that basicly says, "well, if you can't read this, then go here"