The M has significant battery life advantages (like 8 hours with a standard battery) over P4's. They're also usually bundled with WiFi due to the supporting Intel chipset (Centrino?). The clockspeed is slower, but I think most would be hard pressed to show a true performance hit from the slower clockspeed, which is why Intel doesn't list clockspeed with the M's, as it's contrary to the marketing they've been beating us on the head with for the last umpteen years.

-Zeke