Here's the first provider that I have found that is offering the Office Web Apps in a SaaS model.  In this case its Access Services only in a service tailored to help enterprises move their legacy Access DB's and Access applications into the cloud.  This includes conversion of legacy VBA code into Access Services supported macros which can run in the cloud!

There's also some new features in Access 2010 that improve performance when run atop of SharePoint Server 2010.  Check it out the links below for more information -

Check out http://www.accesshosting.com and http://www.access2010converter.com/ for more information. 

Here's a link to a number of video tutorials on using Access Services http://accesshosting.blip.tv/

Here's one on field validation that I found particularly interesting with Access web databases.