HMC 3.5 Feature Pack One includes additional resource management feature capabilities and some partner-requested solution fixes for the Hosted Messaging and Collaboration version 3.5 solution. It is strongly recommended that any service provider deploying Hosted Messaging and Collaboration version 3.5 download and install this feature pack, once they have installed HMC version 3.5.
Microsoft just released Feature Pack 1 for Hosted Messaging and Collaboration 3.5. The "most wanted" feature of Feature Pack 1 must be the new more flexible resource allocation for Mailboxes / Organizations. Prior to this release we we're required to allocate mailbox space at the organization level, but now it can be allocated at the time you provision mailbox users (Which fits most serviceproviders plans/packaging better).
Find the Feature Pack 1 ISO here.