Actually, I think Sought may be right about the guild-to-guild chat mods, though for a different reason: I'd imagine current mods are designed more towards allowing alliances of smaller guilds (10 man guilds who group with others to do 25 mans, and so on) to communicate. That won't be as necessary anymore, since guild perks and the new raiding system will encourage those guilds to merge, so the existing mods won't be as in demand. Calendar mods could be tricky for the same reason.
Of course, the above reasoning has been put forward to suggest that Cata will herald the death of small guilds. Now people are talking about the death of large guilds. This always happens any time anything changes, but the changes usually work out. If there's a significant problem, Blizzard usually addresses it; if they don't, the community does; and if they don't, well, there's always PCGChatter.
On that, while we're looking for a communication option, can raid leaders, and especially people who are putting VoA and weekly raids together, be sure to advertise invites in PCG Chatter? Otherwise people will be reluctant to move their characters until the last minute, for fear they might miss something.