IBM has sold its 20 millionth ThinkPad notebook PC, making ThinkPad the best-selling notebook brand ever, the company says.

Since its introduction in 1992, IBM says ThinkPad has delivered many “firsts,” including the first notebook with each of the following: color LCD panel, 14-inch LCD, full-sized keyboard, and removable hard drive.

ThinkPad’s latest “first” is IBM’s Active Protection System, a new ThinkVantage Technology introduced just last month that helps protect a system’s hard drive, and responds much the same way an air bag protects a vehicle’s passengers.

“ThinkPad is the standard-bearer for notebook computing, and that’s because we listened to our customers from the very beginning,” said Fran O’Sullivan, general manager the IBM Personal Computing Division in RTP. “We listen to our customers. Find out what they like and value from a notebook system, and then we add what we learn to the next round of products. They get better and better every year, but the basic ThinkPad attributes remain unchanged–.”

The 20 millionth IBM ThinkPad notebook, is an Intel Centrino mobile technology-based system featuring 512 MB RAM of memory, a 1.7 GHz processor, a 60 GB hard drive and an extended battery life of up to eight and a half hours by combining the system battery with an optional second battery.

The ThinkPad T41 is one of the industry’s thinnest and lightest 14-inch notebooks, at a little less than one-inch thick and with a travel weight at four and a half pounds.