Well, at the very least, the MS Wireless Desktop has 128-bit AES encryption.
http://www.microsoft.com/hardware/en-us/p/wireless-desktop-2000
"Unlike standard keyboards, this one features Advanced Encryption Standard (AES)..." I don't know if:
1) If by "standard" they mean wireless or wired.
2) If their own "standard keyboards," assuming that they're wireless, don't have encryption.
The Microsoft Wireless Keyboard 800 makes no mention of encryption at all in its "technical spec sheet", so I can't comment on that.
I'm a bit pressed for time so I only checked the MS keyboards because I really love their Natural Wireless Laser Mouse 6000. I recall reading an article a good few years ago that MS keyboards used to use a simple XOR to "encrypt" their data. However, I wouldn't put it past other manufacturers (like the aforementioned Logitech) to do such silly insecure things on their cheap units too.