By the time this question gets 10,000 views (get cracking, publicists!), there will (I predict) be serious plans in place.
The groundswell of popular support for a serif version of the brilliant Ubuntu font will have created a buzz so loud so that with its crescendo, even hard-headed Canonical will bend its ear to popular sentiment, inciting their brilliant design team to crank out another masterpiece.
Designers will immediately proceed apace to their drawing boards and pants-pocket-memo-pads summoning the eudaemons of design and the flickering forces of CAD.
Font-faces will tremble, quiver, and resolve themselves into seraphic serif forms. Small nibs will become the structures that form the serifs on the letters. Ubuntu Serif will be born.
That, or (as Marco implies) someone will fork it. Eight forks and thirteen and a half iterations later, you will have your serif font. (Although it may very well take three pizzas and five energy drinks too.)
Also, here is the wishlist bug item I opened on launchpad . Please add yourself if you are interested (and let me know if I've hashed-up something with the bug report).
On 2/17/11, Bruno Maag updated the wishlist item:
We definitely would love to do it! But
unfortunately, there are no plans,
afaik. I think we need to focus on
driving the current Ubuntu font suite
forward, completing the current set
for May and then expanding the fonts
with language and other support.
Chinese is increasingly becoming a
requirement since successful
localisation depends on a good Chinese
design with comprehensive glyph