Despite the fact that underlining serves the same purpose as italic text, and the fact that in context of web pages underlining is traditionally reserved for marking hyperlinks, it might be sometimes useful to have a third method of emphasis, independent of bold and italics.
The _underlined_ pattern appears very frequently in various text files, e-mails, news posts -- and surprisingly, in a number of many wiki engines using underlined text. Following the technique used for bold and italics in Creole, the underscore characters would be doubled. This gives us an additional markup for Creole: __underlined text__.
Rendering: The actual rendering on the page is somewhat loose, as with the bold and italics. The wiki admin might choose to color the background of the text yellow, for example, instead of actually underlining the highlighted text -- this can reduce the problems of conflicts with link presentation.
Statistics: this (or similar) markup is used generally for various kinds of emphasis by at least 38 known wiki engines. Among them, 8 engines use it for bold text, 7 for italics and 23 wiki engines use it to mark underlined text.
Note: As this markup doesn't seem to be of great importance, and the goal can be achieved easily with different means, it will be probably included as an addition to Creole, not in its core.