I've posted this in another thread, but because this one seems to be becoming the authority on the subject, I'll weigh in here too.
I use an Apple Pro Keyboard, and love it. It's not totally silent, but it is a cut or two quieter than every generic PC keyboard out there. Actually depressing the keys makes close to no noise. However, when the keys aren't being touched, they can wiggle a little bit from side to side, so when you place your finger down on a key or remove your finger from a key, it makes a little bit of noise as the key rocks in one direction or another.
It's also durn sexy. Comes in clear+black or clear+white (I have the black, myself).
It is USB, works great, doesn't need any drivers or anything silly like that, and has two built-in (unpowered) USB 1.1 ports for other devices, which are really convenient for something that you might plug in and out a lot (digital camera, usb drive, etc).
As far as key layout goes, the three keys at the bottom corners of the keyboard are Control, Alt/Option, and Command/Apple, listed from outside to inside. The exact same 3 keys are on both sides of the spacebar. By default, control serves as control in windows, Alt/Option serves as alt, and Command/Apple serves as the windows key. To make the keyboard work like a regular windows keyboard, I added a single line to my registry which works perfectly. I'll include instructions for that at the bottom of this post.
Also, above the numeric keypad, there are buttons for volume down, volume up, mute, and eject. The volume buttons work magnificently within windows, controlling the overall system volume. The eject button does nothing, and I haven't found a way to make it do anything.
Print Screen, Break, and Scroll Lock aren't present, instead there's F13, F14, and F15. Of the three that are missing, the only slightly useful one was print screen, and I survive without it. The F-keys are recognized by windows, so you can set them up to do whatever you want. I have mine controlling winamp (previous song, play/pause, next song).
I've had this keyboard for 3-4 years and it's working fine. None of the keys are sticky or unresponsive. The overall look and feel (aesthetics and ergonomics, to be verbose) are nothing short of the highest that I would expect from Apple.
Registry key to fix Apple Pro Keyboard winkey locations:
add the following Reg_Binary object:
00 00 00 00 00 00 00 00
05 00 00 00 38 00 5B E0
5B E0 38 00 38 E0 5C E0
5C E0 38 E0 00 00 00 00
you have to restart before the fix will take effect.