I did a slight modification to the above instructions on a MacbookPro upgraded to Lion that exhibited this same problem.
In my case, I had "several" iterations of the pref file com.apple.systemuiserver.plist. I had at least a dozen or so variations of this particular plist file going back 3 years. I copied them all to the backup folder just in case.
Made copies of the plist files identified below (including all file name variations) then deleted them from that location and restarted the computer.
I kept backups in the folder on my desktop for a couple of time machine backups beore deleting them--in case I needed to restore them in the future.
Two weeks since and haven't a single problem w/Spotlight indexing as before.
Here are the steps I followed:
1 - Create a Folder on your desktop to save files from step #2 below:
2 - Copy the following files from this location:
~/Library/preferences
com.apple.systemuiserver.plist**
com.apple.spotlight.plist.lockfile
com.apple.spotlight.plist
3 - After making copies of the files and confirming you have them, delete them from the ~/Library/preferences folder.
4 - Empt the trash, then reboot.
5 - If you had Indexer turned off, turn it back on from a terminal session
5 Open the Terminal Application, then use the following commands to turn indexing back on using "sudo" (requires admin password).
sudo mdutil -i on /
Let the Indexes build--can take some time depending on speed of your computer.