This is, according to Bugimus, the Holy Grail and is currently (to the best of our knowledge at the time of writing) impossible as Opera doesn't allow scripting of the iframe.
You should ask yourself why you need an iframe - if it is purely to get a scroller effect then don't use them (we'll address that in another FAQ). The resources listed here deal specifically with loading other pages into your current one which can be achieved using an iframe in IE and NS6 and with load() in NS4.x
These scripts get as close as possible (at least providing the feature in NS4.x):
As of writing this FAQ is number 1 at Google in the search for the terms cross, browser, iframe and the phrase "cross browser iframe" (previously we were number 2 for the terms).