Heirlooms are going to work up until level 80, and Cata will bring new pieces. As for them disqualifying any realm first
achievements I have not read about that for awhile and when I did, it was speculation. I think it would be a little lame
if they chose to disqualify those people in a realm first race like that, even though Blizzard is capable of anything.
Reason being, realms are very established these days and anyone who would even have a shot at a realm first
achievement currently has access to those 1-80 heirlooms. It isn't like some new guy who has never ever played the
game is going to start in Cataclysm and be in the race for level 85 realm first. Does the new guy have a shot?
Technically, yes. Realistically? Not going to happen.

You may want to level up a new character the regular way, to get the full feel for the new cataclysmic changes.