Elven thieves work in all three categories at once (fighter, magic-user, and thief) unless they opt to never be anything other than in the thief category. Thus, experience is always distributed proportionately in the three categories even when the elf can no longer gain additional levels in a given category.With the words 'work in all three categories at once' the elf no longer had to choose what class he would be during an adventure. I have to admit I sort of missed it. Not only because it kept the elf from being two characters at once, but also because it was interesting and unique. A lot of players thought it was odd or inconvenient, or simply just wanted to squeeze as much 'high performance' out of their character as they could at every turn, like it was some sort of ridiculous racing car. They liked the change.
In my Greyhawk PbP game, we've just recently added the Greyhawk supplement, including the revised elf. I miss the old elf, but I also understand it's sometimes hard for a DM to explain. The way I ran it before, the player had to choose between operating as a fighter or a magic-user at the start of an adventure, and remained as chosen until the next safe point was reached, which is also when I awarded experience. From safe point to safe point they were locked in to their choice, whether it was the next day or several days later when they made it back to town.
My question to those reading this: how would you explain it? Suggest a reason why an elf might be forced to make this choice, and then abide by it until next he found himself safe.