strange performance regression between 7.4 and 8.1

Hello,

I have noticed a strange performance regression and I'm at a loss as
to what's happening.  We have a fairly large database (~16 GB).  The
original postgres 7.4 was running on a sun v880 with 4 CPUs and 8 GB
of ram running Solaris on local scsi discs.  The new server is a sun
Opteron box with 4 cores, 8 GB of ram running postgres 8.1.4 on Linux
(AMD64) on a 4 Gbps FC SAN volume.  When we created the new database
it was created from scratch rather than copying over the old one,
however the table structure is almost identical (UTF8 on the new one
vs. C on the old). The problem is queries are ~10x slower on the new
hardware.  I read several places that the SAN might be to blame, but
testing with bonnie and dd indicates that the SAN is actually almost
twice as fast as the scsi discs in the old sun server.  I've tried
adjusting just about every option in the postgres config file, but
performance remains the same.  Any ideas?

Thanks,

Alex