So someone asked Tobold a question, he addressed it in a post (good old Tobold, answering questions from us noobs since... well... whenever he started blogging. lol (Link)
So here's my thoughts on that because I just can't resist. In my opinion there are several ways to handle this. First off would be to continue cutting the time and XP required to level from 1-60 and then 61-70 and then 71-80, etc as each expansion comes out. That's simple.
What about this, give a person who has at least one character at the max level cap (80 for instance when Wrath of the Lich King comes out) a /level command that works up to the previous content level. So when Wrath comes out someone who works their way to level 80 can level their character up to level 60 with said command. That eliminates the alt-grind wait just a bit.
Any thoughts on that? So when the next expansion comes out, Lord of the Maelstrom, for an example that raises the cap to 90 from 80, your leveling command will work to push a character from 1-70. That makes the previous content still active and the new content still active as well.