My guess is that it's launching, then crashing or erroring out in some form. Try sudo launchctl list com.apple.metadata.mds, and look at the PID (id of currently running process, probably 0 since it's not running) and LastExitStatus (which would be 0 if it exited successfully, but may be something else in this case). Also, look at /var/log/system.log (with either Console.app or 'tail') and see if it holds any clues. Also, look in /Library/Logs/CrashReporter for any relevant reports.

As for possible causes: if the logs don't tell you where the problem is, my first guess would be a broken mdimporter plug-in; I'd remove everything (temporarily) from /Library/Spotlight and ~/Library/Spotlight. Also, check /System/Library/Spotlight, and make sure these are the only mdimporters there:

Application Archives Audio Automator Bookmarks Chat CoreMedia Font Image MIDI Mail Notes PDF PS QuartzComposer RichText SystemPrefs iCal iPhoto iPhoto8 vCard

Another possible cause is a badly corrupted spotlight database. If removing suspect mdimporters doesn't get it started, try dismounting everything but the startup volume (& if that solves it, remount them one at a time and see which kills it). If that doesn't do it, try removing the existing spotlight database with sudo mv /.Spotlight-V100 /.Spotlight-V100-disabled (I'm not certain how thoroughly sudo mdutil -E / wipes out the old DB, but renaming it should force a full rebuild).