I/O increase after upgrading to 8.3.5

After upgrading from 8.2 to 8.3.5, the write load on our database
server has increased dramatically and inexplicably -- as has the CPU
usage.

Here's a Munin graph of iostat showing the sudden increase in blocks
written/sec:

  http://purefiction.net/paste/Totals-iostatwrite1-week.png

We expected the upgrade to give us better, not worse, performance. How
do I diagnose this problem?

The application code has not changed, and neither have the usage
patterns. The number of tuple inserts/updates/deletes per second has
not deviated from the normal. However, CPU usage has increased by
around 30%, and the load average has similarly increased.

The other performance metrics I monitor, such as transaction load,
HTTP traffic, etc., show everything else as either normal or slightly
decreased, as you would expect when the database server slows down.

The upgrade was done with dump/restore using "pg_dump -Fc". The old
database lived on a SAN volume, whereas the new database lives on a
local disk volume.

OS is Linux 2.6.24 on Intel (x86_64).

Alexander.