devices+real-keys is subset of devices+soft-keys
I wasn't thinking about about tablets so much as a pocket PDA, about the size of an iPod Touch of Palm Pilot. I see people on the bus flip out a hidden keypad. If you're saying that all those devices also have soft keys, then devices with real keys are a subset of devices with soft keys (at least so far).
If I understood you correctly about this, then I can see why having the absence of soft keys as a requirement could rule out all devices. So for now, I'd like to consider only having real keys as a concrete requirement. Whether it has soft keys or not (and it seems that it will always have soft keys) is immaterial. If that narrows the options too much, then it means that real keys should be a would-be-nice feature (albeit a very compelling one) rather than a mandatory requirement