Apple's annual developer conference in San Francisco.
Verizon Leaving iPhone 5 SIM Slots Unlocked as Required by FCC Network Access Regulations
It's worth noting that though the Verizon iPhone 5 will work on AT&T and T-Mobile's 3G network for voice and data, the phone will not work on AT&T's LTE data network. It will work on a number of international LTE networks. Apple's LTE information page details the bands that each specific model of iPhone 5 supports.
The main reason for the unlocked SIM card slot appears to be FCC network access requirements for Block C, the 700 MHz spectrum that Verizon uses for its LTE network. According to the FCC's CFR Title 47 Part 27.16 paragraph (e):
(e) Handset locking prohibited. No licensee may disable features on handsets it provides to customers, to the extent such features are compliant with the licensee's standards pursuant to paragraph (b) of this section, nor configure handsets it provides to prohibit use of such handsets on other providers' networks.Sprint iPhone 5 models are SIM-locked, but customers can request unlocking after three months for international use. AT&T's existing unlock policy -- to unlock off-contract phones only -- remains in effect for the iPhone 5.