Susan, I think that the fact that the die of most of 3.3V (and in the
future 2.5v & 1.8v) devices are smaller then the 5V devices, can cause
more Single Event Upset (SEU) problems.
"Because the die size is getting smaller the voltage need to be lower"
- S.J.Tompson ;-)
I also think that the original Q. should not be about 3.3V new designs
but on lower supplies as general and their impact of systems designs.
What about 1.8V / 2.5V devices ? Is special layout needed? What about
connectivity between other devices on the system?