The Fn key first debuted on the monochrome display ThinkPad 300 in October of 1992. The Fn key circa 1992 was placed exactly as it is today. Interestingly enough, Apple uses the same positions for their Fn and Ctrl keys as ThinkPad.
Every other notebook personal computer manufacturer that I know of has the Fn and Ctrl key positions swapped. (reversed from how it is on ThinkPad & IdeaPad)
The Fn key was originally placed by the ThinkPad designers in the lower left hand corner to make the key easier to locate when using the keystroke combinations. There was a rationale. This is especially handy for turning on the ThinkLight in the dark. Aim for the two extreme corners. Desktop keyboards have never had the Fn issue to deal with since there are not such stringent size contraints for their keyboards that require the use of such a key.
Newer model ThinkPads support a function to swap the key functions in BIOS. If you have a T410, T510, X201, W510, W701 or later ThinkPad, and wish to remap these two keys, check your BIOS for a setting.
F1 to enter BIOS on startup
Select Keyboard and Mouse
Toggle the Fn and Ctrl Key Swap enable or disable (normal)