Today I FINALLY activated my cell phone in Peru.
Even if you have roommates who have cell phones or think that you will not need a cell phone while you are abroad, I would recommend getting one. They are very cheap and will be very useful when making plans or communicating with others.
It is possible to use a phone from home and get it unblocked so that you can use it internationally, but to tell you the truth- that is a lot of work, and I have found that it is better to simple buy a pay-as-you-go phone here.
I planned on using an unblocked phone from home and then just putting minutes on a sim card while here, but once I got the sim card while in Peru, I found that it was incompatible with my phone. The phone still needed to be activated and none of the companies in Lima are the same or can work with my phone company back home.
All in all- I highly recommend getting a cell phone in Lima. The companies here are Claro, Movistar, and Nextel. They sell these phones in many large stores (grocery store, malls, target-type stores, etc.). My phone, with card and 15 minutes pre-paid cost 59 soles.