Hit by the out of memory killer last night

Last night we were hit by the out of memory killer.  Looking at the following graph, you can clearly see unusual memory growth.  This is a database server running Postgres 9.0.0.

http://mediafly-public.s3.amazonaws.com/dbcluster02-master-month.png

We have another server, running Postgres 9.0.1 which showing a similar behavior pattern:

http://mediafly-public.s3.amazonaws.com/dbcluster03-master-month.png

Compare these to another server which is running 8.3.7 and does not exhibit this behavior:

http://mediafly-public.s3.amazonaws.com/dbcluster01-master-month.png

We're going to accelerate our upgrade to 9.0.2 for these servers, however, I'm concerned that we have not identified the source of the memory leak and this upgrade won't necessarily fix the problem. 

Any advice?  What should I be looking for?

Thanks,
Bryan