Hi Michael,
It's a great question, and the 3 vs 5V issue has long been a problem with Arduino shields in general. We have taken the approach that all of our TinyShields will support 3 - 5V operation. So to do this, we include a local power supply and level translators if necessary on the shields. We're using the TI TXS0102 and TXB0104 translators, and in bulk these are cheap enough to not have much of an impact on the final shield cost.
So the nice thing from a TinyDuino user, is that any of the shields we offer will work with your system powered from 3 - 5V, and you don't need to worry about the voltage of a particular shield, or the possibility of blowing up an SD card due to powering it a 5V, etc.
Thanks,
Ken