slow i/o

Hi everyone,
We have a postgresql 8.1 installed on Solaris 10. It is running fine.
However, for the past couple days, we have seen the i/o reports indicating
that the i/o is busy most of the time. Before this, we only saw i/o being
busy occasionally (very rare). So far, there has been no performance
complaints by customers, and the slow query reports doesn't indicate
anything out of the ordinary.
There's no code changes on the applications layer and no database
configuration changes.
I am wondering if there's a tool out there on Solaris to tell which process
is doing most of the i/o activity?
Thank you in advance.

J