There's no base level or other identifying information on this attribute (we call it a "spell," but really, given its information in the game, it seems like a gear-specific ability), so we can assume that it'll be a full 1-80 bonus, just like the current heirloom gear in the game. Unfortunately, we haven't actually uncovered any new heirloom items yet, so we have no idea if this stuff will show up in patch 3.3 or the expansion. And of course, just because the bonus applies from 1-80 doesn't mean the gear can be worn that whole time -- it sure seems like Blizzard wouldn't necessarily want Worgen and Goblin alts speeding past Cataclysm lowbie content. But we'll see -- in the meantime, look for a little extra heirloom power, coming soon.
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to the Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.