Well, it's taken a few years. I wanted a way to convert between XP and level using an equation, so that it's nice and smooth.
To cut a long story short:
XP = Level * (Level-1) * 500
Level = sqrt(XP / 500 + .25) - .5
Note, btw, that this is not accurate because your ability to earn XP does not scale smoothly - it jumps at the level boundaries. But these equations get the boundary points right, so you can plug them into a spreadsheet or whatever. Don't forget to add a small fudge factor to deal with rounding errors.