Here's some good news for everyone planning to roll a goblin or worgen, or even a human hunter or troll druid, when the expansion hits: Your heirlooms will still work, but only until level 80. At the recent Cataclysm press event, Blizzard confirmed that heirlooms from the Wrath era will still work for new characters in the Cataclysm era, and specifically even for goblins and worgen. Since you earned them, you should still get to use them.
However, they also note that the heirlooms will only work up to level 80. At level 80, you'll have to start using new heirlooms that will be added in Cataclysm itself. So go ahead, buy that Bloodied Arcanite Reaper for your new holy cow or worgen warrior; it'll still work just fine, at least until you get level 80.