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?
-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.