What could lock up pg_stat_activity

Hi,

I have a site with reasonable # of page loads. I log slow selects so I
can get panic later. Today I had a 12s period where things stood still.

As an overload precaution, for each page I...
select count(*) as nbr from pg_stat_activity where current_query <> '<IDLE>'
...to see that not too much is torturing the db. But (roughly) 1/3 of
the entries was this test above. What could possibly make a count() on
pg_stat_activity take 12s? Please speculate wildly.

Setup:
Contemporary Debian
72GB
4x1TB RAID6
2x100GB RAID1 SSD
pgbouncer
private local network
pg 9.0.4

Thanks,
Marcus