That's a common scenario and was a pain in the neck to get results using Analysis Services 2000.
In SSAS 2005 we have.... SEMI ADDITIVE MEASURES: by using this measures we use SUM for all the dimensions but not for the time dimensions (watch out you need to use proper time dimensions!), pretty cool.
Snippet from Projecr Real, technical overview:
(http://www.microsoft.com/technet/prodtechnol/sql/2005/projreal.mspx)
The use of semi-additive measures, as exemplified by the inventory data. Semi-additive measures were not simple to implement in prior releases and did not always perform well. Now the system directly supports them, and their use in Project REAL illustrates this. (For readers not familiar with the term: Semi-additive measures do not sum up over time. Sales for the three months in a quarter can be added to get the total sales for the quarter; this is an additive measure. Inventories for the three months cannot be added to get the “total” inventory—it makes no sense. Inventories at a particular time at different locations can however be added to get a total. Hence, inventories are the classic example of a semi-additive measure.)
Links:
http://blogs.conchango.com/christianwade/archive/2005/04/07/Semi_2D00_Additive-Measures-in-Analysis-Services-2005.aspx
http://www.datadefractor.com/blog/