command line ps has stopped working correctly after an update
I have 10.6.7 and your

ps -u `whoami` -o pid,rss,command


producing

  PID    RSS COMMAND
  180    696 /sbin/launchd
  185  19108 /System/Library/CoreServices/SystemUIServer.app/Contents/MacOS/SystemUIServer
  189     32 /usr/sbin/pboard
  .... etc ....


so, the problem is in your system. So, what return this:

ls -l /bin/ps


should this

-r-sr-xr-x  1 root  wheel  134816 12 okt  2010 /bin/ps


if your ps has other permissions, repair with:

sudo chown root /bin/ps
sudo chgrp wheel /bin/ps
sudo chmod 4555 /bin/ps