For many hard core raiding guilds this means making sure the right members will be on and fully buffed and ready for lots of raiding as soon as Ulduar opens up. Equally important is making sure that reserves and alternates are available so progression raids don't grind to a halt.
For the casual guilds this means ensuring people are aware there's a new patch and some new content to experience, and getting ready to grind out a bit of things folks haven't done before while dealing with an abnormal number of wipes.
Of course, that's just some hasty generalizations about each type of guild – there are many similarities between both types. Either way you look at it, the introduction of a new raid instance means having to get folks ready and organized.
Is your guild prepared for Ulduar?
Mine isn't. /grumbleaboutguildraiding