Note: please check if you're not setting the suid bit on the entire busybox binary. (su might be a symlink to busybox) If you are, then you will in trouble soon. Suggestion: forget about everything said on this thread and just use "root" or "sudo gainroot".
N900:~$ ls -l /bin/su lrwxrwxrwx 1 root root 7 2010-05-11 20:11 /bin/su -> busybox
su - chmod u-s /bin/su