According to a post by Bornakk, the World of Warcraft team doesn't have plans to make the currently available heirloom items auto-update to level 85 in Cataclysm. Throughout the thread, Bornakk explains that "heirloom items specifically state that they go from levels 1 to 80" so you get a lot of mileage out of using them for the first 80 levels. Bornakk does say the team has "thought of providing new heirloom items that must be earned at level 85," although nothing is set in stone at this time.
As for the experience bonus from some heirloom pieces, Bornakk said, "I think it makes sense that we won't want players to get a 20% exp bonus when leveling to 85." In response to a question regarding giving heirloom items to Goblin and Worgen alts, Bornakk said it seems highly unlikely they'd take that option away from players.
With the Cataclysm release date targeted for 2010, it's important to remember that plans can always change regarding heirloom items (and everything else for that matter). Still, what do you think about Bornakk's comments?