The exploit effects the speculative execution facility, so no it's not "all P6 forward": nothing 32-bit or PAE, nothing just OOO, etc. The current word I have (from my risk management folks, who got it from Intel) is the oldest chips verified to be affected are the Xeon 3400 (server) and 2nd Gen Core (desktop) processors. So, probably nothing later than 2009 or so.