This is a new feature of SSRS 2008. Check out - http://msdn.microsoft.com/en-us/library/ms143200.aspx

There are two ways to get around this -

1. Ask users to Click "Last Page" button(From 'report manager'/'report viewer' top toolbar) after report is generated. After clicking this button, total number of pages will appear instead of question mark and it will always remain there if we go to any page page of that report till we close the report.

2. If we put a textbox in header or footer containing "=Globals!TotalPages" (This property is allowed only in header or footer.), the report will display the total number of pages instead of question mark when rendering first page. We can hide that textbox.

Please let me know if this works for you as well.