Is this happening on one table, or several tables?
A quick fix is to bring the system down, drop the index, and recreate it. Try it and see if it stops happening for a bit, anyway.
I dimly recall seeing something about changing in index code in 7.0.2 (I think) causing sporadic errors like this but don't recall details. You might poke around the Postgres site, reading the bugs and hackers e-mail list archives for more clues.
