If you're not already familiar with the Office Web Apps for SharePoint 2010 (Word Services and PowerPoint Services are available in SharePoint Foundation and with SharePoint Server you add Access Services, Visio Services, OneNote Services, Excel Services) here's a several good links

Office Web Apps - end user focused content http://blogs.msdn.com/b/officewebapps/

Office Web Apps - technical content on deployment http://technet.microsoft.com/en-us/library/ff431687.aspx

Office Web Apps - technical content on management http://technet.microsoft.com/en-us/library/ee837419.aspx

So you're probably asking yourself how this related to BPOS? Well in a few ways.  First, with the new release of BPOS being released by Microsoft within the next 6 months or you will have access to the Office Web Apps via your SharePoint Online site collections.  Now we're not sure of all the details or cost information at this point, but it will be a very powerful BPOS feature.  I can't list all the ways having the Word Services or PowerPoint Services would make it easier to modify an SOW document on the fly or present a PPTX right from the web! Second, because BPOS will now embed these services right in your SharePoint Online sites you should be preparing yourself for how you're going to leverage this foundation to build your own solutions.  For example, you could potentially convert legacy Microsoft Access applications to cloud-based databases (helpful tool to do just that).  If you're in a large organization you probably have 50-100 critical access DB's.  In SharePoint you get the scale and power of SQL Server with the UI and Access desktop client integration all in a web interface that your Access DB end users can easily access.  Pretty powerful.

I came across this great solution which can get you most of the way there with your public folder support for legacy SharePoint 2003 to Exchange Online migration scenarios.  It's from a company named MessageOps (http://www.messageops.com/software/sharepoint-bridge) and the product is called SharePoint Bridge.  What it effectively allows you to do is move messages from a designated Exchange Online mailbox into a SharePoint Online document library.  Check it out at the link above.

A frequent question I receive is how to monitor the availability of BPOS and how to access BPOS service notifications.   There's a few good techniques and resources out there.  In my experience, BPOS availability has been great and I've only had one issue since August 2008 when I started using it. It cleared up within a minute or two.  Here's a couple of good tips:

Monitoring BPOS

There's a few different ways to monitor your BPOS service.  If you own a monitoring tool such as Solar Winds' IPMonitor (http://www.solarwinds.com/products/ipmonitor/) you can easily configure fairly sophisticated queries against your various BPOS service URLs such as SharePoint sites or OWA.  I generally configure some alerts againsts my SharePoint sites and OWA.  In the use of your BPOS service you also rely on either your own DNS hosting infrastructure or perhaps a third parties.  I set up QoS monitoring on all my DNS servers - remember if DNS is not working properly mail flow, your website and other services may not function as you have designed.  You can also probably monitor SMTP by configuring a dedicated user (consuming a license) and using an SMTP monitoring rule that can be configured to support authentication (these are available in the IPMonitor product). 

BPOS Service Notifications

BPOS Service Notifications are another great way to keep in touch with Microsoft annouced service issues or maintenance. 

North America Microsoft Online Services Standard RSS Feed https://rss.microsoftonline.com/

APAC Microsoft Online Services Standard RSS Feed https://rss.apac.microsoftonline.com/

Europe Microsoft Online Services Standard RSS Feed https://rss.emea.microsoftonline.com/ 

Message Filtering RSS Feed http://rss.messaging.microsoft.com/