Blizzard's recent developer chat over Twitter
revealed a few interesting tidbits, and this one might have gotten lost in the shuffle of good things. Several of the questions throughout the chat pertained to cross-realm issues, and Cory Stockton and Greg "Ghostcrawler" Street were more than game to answer each one, starting with a question about cross-realm mail (e.g. for BoA items) to which the answer was, "we have cross-faction mail for BoA items now; cross-realm is a work in progress, but we hope to have it done "soon."" This means that it should be possible to send your alts in other realms your heirloom items in the future, which should help along leveling through multiple realms.
That said, it might take a while longer before Blizzard implements cross-realm raiding although they said that they were looking into it. Aside from considerable technical issues, the developers were also concerned about group dynamics, such as how to get groups back together through those long raids that require more than one session. Another concern was keeping the sense of community alive in native realms. On the other hand, the growing culture of PUGs that has bloomed through easier content and the new Dungeon Finder has already diluted the sense of community in individual realms, so we should probably expect Blizzard to allow cross-realm cooperation sooner rather than later.