Recent some one has posted a bug in scanmem, that the program can no longer attach to other processes without run as root. At the moment I was still using ubuntu 10.04, where I couldn't reproduce the bug. Now I've been using 10.10 and I did confirm that bug. So I think this must be something changed in ubuntu. After a brief searching, I found the reason at https://wiki.ubuntu.com/SecurityTeam/Roadmap/KernelHardening#ptrace%20Protection, so that was changed intended, in order to protect the processes.
The key value is the sysctl value "/proc/sys/kernel/yama/ptrace_scope", which is 0 by default before, but now 1 by default.
So in the future scanmem will need to run as root.
And this is quite a good news for ubuntu.