Virgin Mobile is Sprint.
"... Sprint Nextel said Tuesday that it will buy Virgin Mobile USA for $5.50 per share in a stock deal valued at $483 million.
Sprint already owns 13.1 percent of the prepaid mobile operator
... "
http://news.cnet.com/8301-1035_3-10297090-94.html
The deal completed that November. Wisely, Sprint uses a separate entity to market the pre-paid plans and phone to get market segmentation, but it is the same infrastructure.
That's one reason Sprint is right in that it will be a mistake to let Verizon and ATT buy a huge of the spectrum. While it looks like there are multiple carriers out there a large fraction of them are just bulk resellers. They can be toasted at any time if the majors change the terms to being unfavorable.
But yes. Sprint getting the iPhone means that the following year or two it can trickle down to Virgin Mobile and the other operators to leverage off of Sprints base network. ( likewise carriers like US Cellular which leverage off of Verizon can get it too). I imagine that Sprint it going to keep it out of Virgin Mobile line up until they make good money off the iPhone on their subscription plans. Besides without a contract "credit payment plan" (subsidy) the iPhone are a bit expensive for contract free ( for the US market conditioned consumers. )
No Sprint will have to allow any iphone on Virgin Mobile as AT&T and T mobile allow the iphone on there prepaid plans.