Are you sure about this?
I tried replicating this in SSRS 2005 (unfortunately I don't have SSRS 2008 installed on my machine).
With a dataset like:
Select 'Pre Order #1' as Text
union
Select 'Pre Order' as Text
And setting it for a table, I get two rows as expected.
Well, I've seen a case where if column name or alias contains #, it is replaced with an underscore.