It depends on which hosted exchange provider you will be using.
Microsoft Online Services has Active Directory importation and synchronization. And they are quite reasonable in terms of pricing ($5 per user per month i think?). It imports not only the users names, but all information stored in Active Directory. There are many hosted exchange providers out there that will not offer this feature (like the one we are currently using AppRiver Hosted). Your best bet is to open up a dialog with your proposed provider and see what services they offer in terms of migrating from your physical exchange to hosted exchange, and also what features they offer in terms of interactment with your current environment. So far, I have found Microsoft Online services to offer the best features and tools to make the move the easiest.
Right now these "Cloud" providers are in a frenzy to gobble up customers, so you will often find them more than willing to bend over backwards and put up with every and any question you may have. As they should regardless.
