Raid progression in Wrath - How?

Q u o t e:
Will 25 man guilds have to do 10 mans first or will it be possible to go straight into 25 mans and only do 25 mans? Or will we have to zig zag back and forth?

You can do them both and neither is required for progression in the other. Though there may be some additional thought put into being able to engage Arthas via 25-player prior to it being available to 10-player, but that is still in debate and just one of the ideas being toyed with.