“What keyboard areas are “sacred” to you, as a developer?”
My Visual Studio keyboard mappings (esp. function keys)!!! And the PgUp, PgDn cluster, as you highlighted (I used a Dell USB keyboard with the new ‘trendy’ 2x3 rather than 3x2 cluster - it lasted ~ 30 seconds).
Muscle memory is very important when you have a strong link to a particular environment. When I telnet onto our VAXs, I use a VT-330 keyboard layout, even though they use PgUp, PgDn cluster completely differently to PCs. When I’m using the VMS Eve editor, my muscle memory is strong enough to remember that the ‘Insert’ key is the ‘Select’ function, ‘NumLock’-‘PgUp’ is ‘Copy’ etc…even though I’ve not used VMS on a day-to-day basis for 7 or 8 years.
“How important are the extra multimedia functions such as sleep, calculator, scrollwheel, volume control, etc? Do you use this stuff frequently? Rarely? Never?”
I have toolbars for stuff like that. I use them occasionally for things like the web browser if they’re available.
“Does it really matter if your keyboard is wireless?”
I’ve never used wireless input devices and I think I’d prefer not to. My keystrokes are too important to lose in the ether 
“Does an ergonomic layout help?”
Not IMO. I’ve only ever used standard keyboard layouts on a day-to-day basis, and others don’t seem quite right.
“What keyboard(s) do you recommend, and why?”
The best feeling keyboard I’ve used was an old Digital one, attached to a VAXStation 4200, IIRC. Having said that, I use a Logitech one at home and a Dell one at work, and am quite happy with both. I’m even quite getting to like my Dell laptops keyboard.