ILM 2007 delivers ExchangeUtils class which can be used to provision new Exchange enabled objects into Active Directory environment. This is nice and easy to use API, however it has one disadvantage – it will not allow to provision mailbox for existing Active Directory account.
As this isn't preferred way to do this sometimes You have to provision users first and then mailboxes. If you have to do it in Exchange 2003 environment You can use information provided in old KB 275636 article, which discuss how to provision mailboxes with MMS. Just build Your attribute flow to populate these attributes and RUS will take care about the rest.
What is not mentioned in this article and what You may want to do is to set additional attribute mDBUseDefaults which when set to True will indicate that store quota setting should be used for this mailbox. This isn't mentioned in this article but will probably be required by Exchange administrator You will work with on such implementation.
And remember – test it first in the lab :).