The Kensington Numeric Keypad I purchased for my 12" PB through Apple.com would flicker the number of the key depressed but then disappear. It worked fine in Virtual PC mode.
After much frustration, a second keypad shipped courtesy of Kensington, many phone calls, and drives about town, my third call to Kensington paid off.
After plugging the keypad into the USB port, depress and hold the NumLock key for 5 seconds or more. It then works as advertised. Don't know why, but glad it finally works.