[zach]

The first few games to use an exponential XP curve may have copied each other, but by now basic things like this are simply conventional wisdom in game design.

Here's Diablo II's incremental XP vs. level graph — essentially identical: http://hothardware.com/articleimages/Item1824/D2-XPScale.png

[treysmith]

See my other reply, it wasn't just XP and Level. Earn Rates per minute, cost of items per level, etc was also similar.