Yes it can be unlocked just like any other BB, but you have to stick with iDen networks as stated before. Some users unlocked them for the Boost network, although I do not see much of a point there as you can't get BB services from Boost.
Anyways, here's a site that has it for $25. GSM phone source will also do it for $60. How to Unlock RIM BlackBerry 7520 Unlock [7520-Unlock]