Hi Christopher (and everyone else affected by this issue),
I'm not totally sure I'm having the same issue as everyone else since my Spotlight seems to have never indexed after I upgraded from Snow Leopard to Lion. I discovered the problem when tyring to set up a bootcamp partition and finding that Finder showed 90GB free and Disk Utility (and consequently bootcamp assistant) showed only 68GB free of 250 on my 2009 MBP. On the advice of Apple tech support I forced a reindex (or I guess maybe just index) of my HDD. Adding and removing the entire disk from the Privacy tab on Spotlight did not start an index so I had to start it through Terminal. Since starting the indexing, Spotlight has moved from 3 days to now up to 2 weeks. (FWIW, I could/can still make TM backups to my external drive just fine).
I'm trying your fix by adding /private/var/folders to the Privacy tab on Spotlight. Simply adding the directory did nothing. Spotlight continued to report "About 2 weeks" to index my HDD.
I then went in to Terminal and disabled indexing using the two commands you suggested (1. sudo mdutil -i off / and 2. sudo mdutil -E). I then restarted indexing using sudo mdutil -i on.
It has been approximately ten minutes and Spotlight is still estimating the time it will take to complete the index.
I will post here again to let you know if it completes the index or not. If it does not, I will try deleting the priate/var/folders directory and then restarting the index and post my results.
If anyone wants more information about my particular system/problem, just let me know and I will post.
Thank you all for your help (especially Christopher)