Well, here's some good news for iPhone addicts. Reports are now surfacing that such a conversion is possible by carefully cutting down the bigger SIMs. And although nano-SIMs are thinner than the larger SIMs, the iPhone 5 socket can also accommodate a cut-down SIM, which means thinning down the card is not necessary.
To cut your SIM, either use a special nano-SIM cutter or follow the instructions in this post.
My recommendation for B-Mobile and TashiCell: Until (and if) nano-SIMs are procured directly, equip the sales offices with nano-SIM cutters.