It's pretty nifty and subtle enough in my opinion. There is no way to disable/hide the XP bar. It is currently bright yellow, but if anyone of you want to take a shot at making it look better, just edit xpbar.png located in the basicdat folder (update to latest SVN!). It is basically like editing the other status bars... just that it's half the size.
The XP bar has been in the game for half a year now... since after 2.7.4 was released I think (meaning it just didn't make the last release). The upcoming 2.8.0 has the XP bar.
I really like that XP bar. Ingame, it is small and subtle, and seeing it without pressing the stats key is actually a good and useful thing, especially in modules with multiple characters, like Adventurer, Rogue and Soldier Starter.