Version 2008 and earlier don't have this feature, see this Microsoft Connect issue.
For SSRS 2008-R2 you should be able to do this, from the MSDN tutorial:
- Create a new report or open an existing report.
- If the Properties pane is not open, click the View tab and select the Properties check box.
- Click the text box for which you want to change text orientation.
- Locate the WritingMode property in the Properties pane and in the drop-down list select the text orientation to apply to the text box.
- Note: When the properties in the Properties pane are organized into categories, WritingMode is in the Localization category.
- In the list box, select Horizontal, Vertical, or Rotate270.
- The Rotate270 value is the option you're looking for.