Why does Spotlight keep reindexing my system after updating to Lion?
I, too, had the reindexing problem happening after I upgraded to Lion. However the problem didn't exist for every user, just for the one that was using FileVault 1.

After a few weeks I got overfrustrated and ran… †

$ sudo mdutil -a -i off


… in Terminal — and the same minute I realised that the setting was applied system–wide. I shrugged and enjoyed a few hours worth of reindexing–free session after which I decided to turn Spotlight back on, as it is crucial to my workflow on other user accounts; so:

$ sudo mdutil -a -i on


And while I was at it, I decided to erase the old index and rebuild new from scratch with:

$ sudo mdutil -aE


I did this about a month ago and I haven't encountered "useless" reindexing since. I'm happily surprised because fixing the issue was merely a coincidence.

So, have you tried turning it off and on again?



†) the -a flag will apply the action to all volumes  which might not be something you want — and actually wasn't what I wanted either, but I just wanted to go blitzkrieg on the issue.