Longer and longer updates

Hi all-

In doing some stress testing I noticed that our database was slowing
down significantly after doing the same update repeatedly on a table
with no indexes, so I tried this:

1) created an empty database

2) made a simple table with a single row:

   create table test (key int4, data int4);
   insert into test values (1, 0);

3) executed this statement tons of times:

   update test set data=1234 where key=1

Here are the results -- it's pretty discouraging, I hope I'm making some
simple mistake, or maybe this is expected behavior for some reason?

After this many updates        ...it took this long for 1000 more updates
-----------------------        ------------------------------------------
        0                                   10880 ms
      5,000                                 10549 ms
     10,000                                 17380 ms
     15,000                                 20040 ms
     20,000                                 20060 ms
     25,000                                 20589 ms
     30,000                                 30749 ms
     35,000                                 30350 ms
     40,000                                 30910 ms
     45,000                                 37570 ms
     50,000                                 40379 ms

This seems to be independent of starting and stopping my client and the
postmaster, running vacuum, praying, etc.  I'm on RedHat6.2
running with the 7.1beta4 rpms.

Anyone know what's going on here?

Steve Wilmarth