Multiclass penalty

If a multiclass character's classes are not nearly the same level, then he suffers a multiclass penalty of -20% XP for each class that is not within 1 level of his most experienced class. These penalties apply from the moment the character adds a class or raises a class's level too high. For instance, a 4th-level Wizard/3rd-level Rogue gets no penalty, but if that character raises his Wizard level to 5th, then he'd receive the -20% penalty from that point on until his levels were nearly even again. The penalty is -40% if all three levels are uneven, for instance, a 5th-level Wizard/3rd-level Rogue/1st-level Fighter.

Races and Muticlass XP
A racially favored class does not count against the character for purposes of the -20% XP penalty. For instance, an 11th-level Gnome character (a 9th-level Rogue/2nd-level Wizard) suffers no XP penalty, because he has only one nonfavored class (Wizard is favored for Gnomes). Suppose he then achieves 12th level and adds a level of Fighter to his classes, becoming a 9th level Rogue/2nd-level Wizard/1st-level Fighter. He suffers a -20% XP penalty on future XP he earns, because his Fighter level is so much lower than his Rogue level.

A Dwarf 7th-level Fighter/2nd-level Cleric suffers no penalty, nor does he when he adds 1st-level Rogue to his classes since his Cleric and Rogue classes are only 1 level apart, and Fighter is a favored class for Dwarves.

A Human or Half-Elf's highest-level class is always considered his or her favored class for purposes of the multiclass penalty.

Human is not always the best choice to avoid an XP penalty: For example, a Human 2nd-level Fighter/8th-level Wizard/8th-level Rogue would suffer a -20% XP-penalty, a Dwarf with the same class combination would not. Humans and Half-Elves can never suffer a -40% penalty though.

Prestige classes and Multiclass XP
Prestige classes are exempt from the Multiclass XP rule and do not count against the character. For example a 20th-level character (10th-level Rogue/3rd-level Shadowdancer/7th-level Assassin) will not suffer any XP penalties no matter what race the character has chosen.

Pre-release notes
Obsidian have not confirmed what prestige classes with be included in NWN2.