How do I find the bottom 10 customers with the lowest sales in 2003 that were not null?

A: Simply using bottomcount will return customers with null sales. You will have to combine it with NONEMPTY or FILTER.

SELECT { [Measures].[Internet Sales Amount] } ON COLUMNS ,
BOTTOMCOUNT(
NONEMPTY(DESCENDANTS( [Customer].[Customer Geography].[All Customers]
, [Customer].[Customer Geography].[Customer] )
, ( [Measures].[Internet Sales Amount] ) )
, 10
, ( [Measures].[Internet Sales Amount] )
) ON ROWS
FROM [Adventure Works]
WHERE ( [Date].[Calendar].[Calendar Year].&[2003] ) ;