So I hacked around in sp_spaceused and got this sorted out. Works in SQL 2005, haven't tested north or south of that.
Share and enjoy.
drop table #mainpartitionstats
drop table #LOBPartitionStats
drop table #totals
SUM (reserved_page_count) reserved_page_count,
SUM (used_page_count) used_page_count,
WHEN (index_id < 2) THEN (in_row_data_page_count + lob_used_page_count + row_overflow_used_page_count)
ELSE lob_used_page_count + row_overflow_used_page_count
WHEN (index_id < 2) THEN row_count