Getting Excel Web Access Errors with SSAS 2008?
If you are having issues getting Analysis Services 2008 data being refreshed through Excel Services, take a look at the trusted providers within MOSS. By default, MOSS only trusts up to SQL Server 2005, simply because SQL Server 2008 wasn't out when MOSS 2007 was released.
To correct, you'll need to add a Trusted Data Provider as follows:
Provider ID = MSOLAP.4
Data Provider Type = OLE DB
Description = Microsoft OLE DB Provider for OLAP Services 10.0.
For full explanation/steps that walk through the "fix", follow this link.