The formula is
XP = A*(XP previous level) + B*Level + C
A = Previous Level Factor
B = Level Factor
C = Constant Factor
You can define values for specific levels at Expierience Gained - Table. I mean... The first value in the list would be the XP for the level 1 (and lower), and so on. If there's no value for the level, the formula will be used...
Don't forget the creep reduction =P
Hope it helps ^^
(I found that at Blizzard's site, by the way xD)