It'd nice to have a dual sim tray feature on the iP6 so you could just pop in a second sim when traveling abroad, maintain two contact numbers, and not loose the one for home since they're so tiny.
they've already gone for the smallest SIM they can have, they won't add a second slot. It was the reason that the iPod touch had the extra storage, the space taken up by the SIM slot was used for a second storage chip. I'm sure they'd love to go for a software SIM if the operators would go for it.