One of the problems I've run into is the size of the fact tables.  This is with a with a web site doing 70+ million hits per year.  I've found that the majority of reporting works on month/year as the major time dimension, so I created a function that changes the date to the first of the month.  I don't like the idea of chosing a particular day, but OLAP tools are looking for a date and not just a month/year combo.  This cuts the size of the data into 1/30th of what it would be otherwise. 

CREATE FUNCTION [dbo].[GetRptDate] (@Date datetime) 

RETURNS datetime AS 

BEGIN

DECLARE @StrYear nvarchar(4)

SET @StrYear = Year(@Date)

DECLARE @StrMonth nvarchar(2)

SET @StrMonth = Month(@Date)

RETURN  convert(datetime,(convert(nvarchar,@StrYear+'-01-'+@StrMonth, 103)), 103)

END

I use this function as part of an ETL job that runs daily.  There is a lot more to it than this, but I wanted to see how other people are dealing with VLFT (Very Large Fact Tables - a new acronym?). Thanks.