I recently went to a days training on BPOS thanks to the MSFT partner program.
BPOS is basically MSFTs entry into the hosting market ad actually looks pretty good! I was expecting something that was very rigid but actually it seems like a fair amount of customization can be done, especially in the SharePoint area.
Below are a bunch of resources which might help if you are investigating it.
Microsoft Exchange Online Standard
Microsoft Online Services TechCenter
Microsoft Online Services Webcasts
Microsoft Online Services Home Page