My IBM Thinkpad Type 2611-411 says it has a memory limitation of 256
MB. The system was running Windows 98 but I have upgraded it to
Windows XP. It works fine but I would like to override the maximum
memory and increase it to 512 MB.
Anyone know how this can be done?

The problem isn't the software itself.  The main issue is the
motherboard being ed in the laptop.  In order to upgrade above the
256MB limit, a new motherboard would have to be installed into the
laptop; which, you would be better off buying a new laptop.

Unfortunately you won't be able to upgrade that notebook to more than
256 MB of RAM because of a hardware limitation.  The chipset wasn't
designed to handle any more memory.  In fact, there is likely only 28
addressable traces (2^28 = 256m) on the mainboard between the
northbridge and the DIMM socket, so even if you stuffed a bigger DIMM
in there it would only be recognized as 256 MB.
Also, be aware that IBM is recalling the power adapters that shipped
with most of those notebook computers.  Check this URL to see if
you're affected:
http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=TPAD-RECALL