Firefox 4 already runs plugins in separate processes. Running each tab in its own process is codenamed "electrolysis" for reasons beyond my ken: https://wiki.mozilla.org/Electrolysis
I'm guessing Electrolysis will get some more loving once the panic of shipping Firefox 4 has abated.
http://en.wikipedia.org/wiki/Electrolysis is a chemical process that can be used to separate out elements -- picking that as a codename for splitting up tabs into processes seems reasonable.
I'm guessing Electrolysis will get some more loving once the panic of shipping Firefox 4 has abated.