I noticed huge (~ 500 Mb) usage of memory on my workstation running on Ubuntu 9.10 (on amd64), causing issues when running large numerical numpy scripts. It ended up being related to ureadahead.
Removing ureadahead, and rebooting seems to fix the issue for now. I am baffled such a bug stayed in a release version. It seems that every new version of Ubuntu is slightly worse than the previous one. It is still better than many other low-hassle distributions, but I worry about the trend.