Developed as part of Google Summer of Code 2007, this was an idea to develop some features that will improve the usability in the Gnome Desktop.
The goal is to implement a Desktop Voice Control System. The system
consists in an application that will be monitoring the audio
input(microphone) and when a significant audio signal has been
detected, the software catches, processes and recognizes the signal
and then executes the desired action over the Gnome Desktop. In a set
of actions could include maximize, minimize, close the active window;
open a specific program; switching from one desktop to another; among
others. GnomeVoiceControl is implemented in C in conjunction with CMU
Sphinx, which is an open source tool, created to convert speech to
text.
Slide-show by the authors
Unfortunately it looks like it has not developed much beyond the basic "goals" - however there does appear to be some activity adding support for new languages such as Bulgarian and Turkish this year.
To install:
sudo apt-get install gnome-voice-control
Platypus
There is an interesting looking project call platypus - basically is a linux front-end to Dragon Naturally Speaking which runs in Wine.
The claim-to-fame for this application is "it can even launch Windows or Linux programs and scripts, e.g. "start terminal".
VEDICS
This is a newish project that says it works with the Unity interface.
The project page says it recognises "run [program name]" - so in theory you [program name] could be any script you want to run.
Best Answer
No, with the current set of software for Ubuntu, it is not possible to login by voice.
There is no software to allow you to do this. Voice recognition is still a very new and imperfect technology. Everyone can agree it would be cool and high-tech to walk up to your computer, say something, have it say "Access Granted" and log us in. But right now, we can only dream.
To create a secure voice login would require an extraordinary amount of computer power to accurately analyze the voice. Apple's Siri sends the data to servers to have them report back what was said, because it does not have the power to do it on it's own.
However, I think it would be extremely cool and is a great idea. If you would like to get the ball rolling, head over to http://brainstorm.ubuntu.com site and start the discussion there. You'll get feedback from lots of people including Ubuntu developers, and there if enough people think it's a good idea it'll happen.