
the new penryn can probly do 4.0 on air with unlocked multiplier

and the number of applications which are putting to use 4 cores is growing every day. new games, new apps, encoding, decoding video, music, etc, all take advantage of the extra cores (if you are using decent software)
in a year, quad core will be completely mainstream, as there are quad cores already selling in normal retail stores (most accessible to the general public)