Pretty impressive what cell phones hardware are capable of nowadays... it was only a few days ago that Sega announced they would be developing games for cell phones and now this.
This site (WildPalm) is distributing a Nokia 7650 version of the classic first-person shooter at no charge for a cut down version or for $10 your get the fully functional shareware version of the original Doom working on your cell phone. The download is aprox. 1.5mb, further instructions can be found here.