Several confusing pieces of information here.
First, the link to Virgin Mobile in the first post is for Virgin Mobile Australia. They will unlock their phones if you ask them.
Virgin Mobile USA is a different company (in a different country) and does NOT unlock phones.
Second, the iPhone sold on Virgin Mobile USA does have a SIMM card slot, even though VM network is CDMA. These iPhones are hybrids and have radio circuitry that supports both CDMA (Verizon, Sprint, VM USA), as well as GSM (T-Mobile, AT&T in US, as well as the rest of the world). When activated on Virgin Mobile, phone uses CDMA and the SIMM card is inactive (as well as the circuitry supporting it). However, when you put an active SIMM card (for example, T-Mobile's), the phone should be able connect to a GSM network. The problem is, this doesn't work because the SIMM slot is locked and doesn't allow SIMM cards from other carriers.
Virgin Mobile iPhones are somewhat different from other American carriers, and it appears that there is no unlocking solution for these. I have searched the web thoroughly and haven't been able to find anyone who would be able to unlock an iPhone sold by Virgin Mobile USA, so that it can be used with a SIMM card of another mobile carrier (domestic or foreign). For iPhones on Verizon, unlocking isn't even necessary; they are CDMA phones, which means that the CDMA circuitry can only be activated on Verizon (therefore no Sprint or Virgin Mobile), but the SIMM slot is unlocked and can be used with any global carrier. With AT&T and T-Mobile, unlocking is required, since the two are GSM operators which use SIMM cards, so the phone is locked to the carrier. Unlocking can be done, in some cases (if you've been a customer long enough) even by the carrier itself, and there are third-party unlocking services out there. Unfortunately, Virgin is the only one where this, for some reason, cannot be done.