The Verizon iPhone 5 is GSM-unlocked and has been tested successfully on the AT&T network, according to a post at iDownloadBlog, which confirmed that a phone purchased under contract from Verizon worked on the AT&T HSPA+ network. For purposes of testing, the user cut down a micro-SIM card and placed it in the iPhone 5, but nano SIMs from AT&T and T-Mobile should also work in the Verizon phone. While reports have suggested that Verizon’s…