Inside Sprint Now, Virgin Mobile's iPhone 4S will have a locked international SIM slot. Which means the phone will only work in the US. Which is a pretty lousy way to treat customers spending $649 for what's supposed to be a "World Phone".
The version of the iPhone 4S sold by US CDMA networks including Sprint and Verizon has a dual mode radio that supports both CDMA and GSM networks. That lets you use the phone outside North America where CDMA is mostly non-existent.
Both Sprint and Verizon's iPhone 4S's come with a roaming SIM that lets the phone work almost anywhere in the world. That's convenient but it's also insanely expensive. Sprint charges anywhere 59¢/minute for calls in Canada to $2.49 in India. Data roaming is equally overpriced; $19.46/MB in India and $2/MB in Canada.
Experienced international travelers generally use an unlocked phone and either buy a local prepaid SIM in each country they visit or use a global travel SIM from companies like Tru or Maxroam. Sprint and Verizon's iPhones do come SIM locked, but both operators will unlock their iPhone's GSM side for postpaid customers.
Given that Virgin Mobile's 16 GB iPhone 4S costs the same $649 as buying an unlocked one directly from Apple, there's been some speculation that the Virgin Mobile iPhone would be sold with an unlocked SIM slot. However, Inside Sprint Now has published a Q&A document intended for Virgin Mobile support personnel that states Virgin Mobile's iPhone 4S will have a locked international SIM slot.
"Q: Is the International SIM locked or unlocked for Virgin Mobile iPhone 4S customers? iPhone sold by Virgin Mobile will be locked to their network and will not be able to be used when located outside the US"
That "...will not be able to be used when located outside the US" statement seems to indicate that Virgin isn't going to be including a roaming SIM and won't be unlocking the iPhone's GSM functionality either. I really have to wonder why. If the iPhones where being sold a subsidized price, Virgin would want to lock them to discourage people from buying them for profitable resale abroad. But as they are being sold at full price there's no incentive to buy a Virgin iPhone except to use it on the Virgin network. By not offering to unlock iPhone's, Virgin is forcing it's users to purchase a second phone for international use, and for no good reason that I can see.