Microsoft Exchange 2010 includes a new (and great) feature, called online archive, which enables you as an IT administrator to provision your mailbox-enabled users with a familiar personal archive, available online, providing you with a lot of features to preserve and discover e-mail data.
And there is Microsoft Exchange Hosted Archive, which is available now, and can be summarized as "a centralized, easily accessible, and multi-functioning e-mail and IM repository to assist organizations manage increasingly complex retention, compliance, e-discovery, and regulatory requirements" (as taken from the MsOnline site).
If you want to know more about the archiving features included with Exchange 2010, head over to the following site:
And if you want to know more about Exchange Hosted Archive, head over to this site:
To answer a very recurring question: can we upload historical data to Exhange Hosted Archive?
The answer can be found in this document: