Hardware

Some CPUs are soldered down (to save money as sockets are expensive)
so those would not be upgradable,the BIOS has to support it as the voltages are different. So yes, it is possible but not a given unless you know more about the particular model. I have a thinkpad R40 with a 1.5 Banias cpu, I know it is socketed, but I'm not sure about the BIOS. I'll wait a year or so for the prices to drop and take another look at things. It may be that by then there are other things that I would like to upgrade and it might just be a better deal to get a whole new notebook.

IBM offered the T21 with CPUs up to 1GHz.

http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=MIGR-4PEJ5Q

Intel has instructions for how to change the CPU "IF" it is socketed.

http://support.intel.com/support/processors/mobile/pentiumiii/sb/CS-007504.htm

IBM does not offer a CPU upgrade on their web site.
It might be worth checking with IBM tech support to see if it is possible to upgrade your CPU.

Since the 1GHz CPU would be 25% faster, the best you can expect is a 10 to 15% improvement in performance.

Before spending money on a new CPU, you get the best performance gain for the money by increasing RAM.
1 caveat, if you are running Windows 9X or Me, it can cause problems if you install more than 511MB of system RAM.

Chas