Below is an excerpt from the Microsoft Online Services Team Blog
Upgrades and Enhancements
The following features and capabilities are included in this Service Update:
Bulk Activation of Users
Commandlets added in this Service Update are:
POP Mail Clients
Supported e-mail clients for Exchange Online are being expanded to include POP-based clients. Previously, Exchange Online supported only Microsoft Office Outlook 2003 and 2007.
Support for 30,000 Users
The Microsoft Online Services Administration Center will support as many as 30,000 users, improving management and administration for larger organizations. Previously, Microsoft Online Services supported up to 10,000 users.
Enhanced Support for Mac OS
The Service Update includes several enhancements that will provide a more efficient communication and collaboration experience for organizations with Mac users.
For the full October Update go to the Microsoft Online Services Team Blog http://blogs.technet.com/msonline/archive/2009/10/14/service-update-october-2009-release-availability-features-announced.aspx