Pre-installed 'ps' for BusyBox really sux, but nice solution is to install 'procps'.
Read more to find out, how..
~ # ipkg install procps
Installing procps (3.2.8-1) to root...
Downloading http://ipkg.nslu2-linux.org/feeds/optware/cs05q1armel/cross/unstable/procps_3.2.8-1_arm.ipk
Configuring procps
update-alternatives: Linking //opt/bin/free to /opt/bin/procps-free
update-alternatives: Linking //opt/bin/kill to /opt/bin/procps-kill
update-alternatives: Linking //opt/bin/ps to /opt/bin/procps-ps
update-alternatives: Linking //opt/bin/top to /opt/bin/procps-top
update-alternatives: Linking //opt/bin/uptime to /opt/bin/procps-uptime
update-alternatives: Linking //opt/bin/watch to /opt/bin/procps-watch
Successfully terminated.