You can Google "PPC cloaking" and get quite a few interesting results. There are also professional cloaking services that maintain large IP datasets. The two most popular of those are called Just Cloak It [1], and, ironically, FraudBuster [2]. Also if you're looking in general for information on this and other black/grey hat marketing techniques, Black Hat World [3] is a great place to start.

[1] http://justcloakit.com/

[2] http://fraudbuster.im

[3] http://blackhatworld.com