As far as I know iframes do not affect indexing or ranking in most cases. For example, the content of an iframe still doesn't seem to be attributed to the URL that holds the iframe, because the iframe content lives on its own URL.
That said, if you use an iframe to push your actual content out of easy view of the visitor, Google's page mock-up algorithm will see that, and it could be one negative issue that Panda scores on.
Also note that Matt Cutts said Google may change the way iframes are scored in the future. However, I'm pretty sure that would not be for mere ad serving. Iframse for serving ads are quite common.
Does Google treat scrollable content within a page the same as iframes (the content is only on that page)? I'm not familiar with iframes at all, but need to use scrollable tables to minimize page size for the viewer...
The content of a scrollable element is actually on the page's URL and naturally has always been scored as part of the page. The content of an iframe is on a DIFFERENT URL, and historically has not been scored as part of the parent URL's content.
Google has suggested that this may change in the future, at least in some situations - but so far no one has reported such a change.