Once you hit 70 you get gold based on the ammount of XP you would have gotten. If you want to go around and get a bunch of gold from old outland quests before you upgrade to wotlk this is the time to do it.

If you leave a bunch of level 70 quests in outlands and level to 80 then come back. Some of these quests will be gray to you thus giving you no xp (or extra gold).

So I think the point is if you want to get the maximum ammount of gold before northrend do the quests before upgrading your accounts.

Also moosejaw will be earning more gold that they can put toward other things besides mounts from leveling 70-80. The real problem with epic mounts is as soon as you get one you can't stand to not have them on all the toons you are playing.