Italic sarcasm is saying a statement with a heavy sarcasm on words such as she, he, that, not and other major words (a higher pitched voice than usual is also necessary to get full effect). It is usually used to make fun of people, primarily in comebacks. When trying to use this style of speaking on aim, one usually puts the sarcastic word in italics or caps.
Sitting in Spanish class...
Mark: Hey Matt you suck at Spanish.
Matt: oh YOU'VE got friends.
Mark: NOT gay
Observers: Wow, Matt and Mark having a pretty bad italic sarcasm fight.