Ubuntu features quite a professional font, Ubuntu font, but lacks a monospace flavor that is to arrive in the current developemnet Ubuntu release or in a future one.
What is the state of Ubuntu Monospace font?
First of all, the actual work on Ubuntu Mono font started in August 2010, and, perhaps this long period of time triggered some misunderstandings/concerns or "When will we see Ubuntu Mono on our Ubuntu machine?"-related questions.
Ubuntu Mono is expressed via 4 flavors: regular, bold, italic and bold-italic, that are true monospace "0.5em wide and 1.0em high, regardless of the weight".
"Making a font takes a really long time, for Ubuntu Mono it has also been necessary to re-learn and re-discover much of a lost-art behind monospace font design", but there has been, in the last months, a private (as opposed to unregistered launchpad users) testing PPA that allowed folks to track and test the font's development.
Ubuntu Monospace font has just reached another development stage, bringing numerous improvements and extended support, still hasn't yet landed in Ubuntu's main repos.
Will Oneiric Ocelot feature Ubuntu Monospace font?
"Finally, remember that the Ubuntu Font Family is about quality, it’s better that we all get a high-quality monospace font in the long-run than to rush something out of the door too soon. Good things are worth waiting for!" (so, probably, Ubuntu 12.04)
To test and give feedback, you can apply to the ubuntu-typeface-interest team and follow the PPA instructions after being accepted (could be a matter of minutes).