Music Thing is running a story covering the history behind the Mac Startup sound, and they even have a link to an mp3 of the actual sound file [file link].
The sound was created by Jim Reekes, the same guy who invented Sosumi. According to Reekes, "The startup sound was done in my home studio on a Korg Wavestation. It's a C Major chord, played with both hands stretched out as wide as possible (with 3rd at the top, if I recall). This just sounded right to me. I wanted something really fat, heavy bass, high notes, and a sharp attack. The chiffy sound was from pan pipes and something like a stick hit (I'm testing my memory here). I wanted lots of evolving timbres, stereo phasing, and reverb for further richness."
And, finally, here's my lame attempt to replicate that signature sound via type: BuhWAAAAAAAAH. Now you try!