I do not see how legacy apps (i.e. x86 binaries) will run on ARM-based windows tablets in any meaningful way. While they could in theory use x86 emulation, it would be very slow and power-hungry, I just don't see it being practical, so you'd have to wait for ARM versions of your existing apps.
Just following up on this, since Microsoft has finally seen fit to stop dicking about and confirm it; there will be no Windows 8 ARM support for x86 apps. So it seems like Windows 8 ARM devices will indeed be very much the second class citizens of the Windows world, unable to do the one thing that might have given them an edge over Android and iOS tablets, i.e. run existing Windows apps. It was obvious, but the way Microsoft has danced around actually admitting it says to me that they realise it's a big problem for them.