This didn't work for me (Word 2010, Win 7, 64 bit) but this does;
Enter Word art text as you wish, then
- select the text you wish to rotate
Hi,
Mirror image of the content can be obtained using the following steps.
1) Insert a WordArt and type in the content.
2) Now click on the WordArt.
3) Go to the “Format” tab under the contextual menu “WordArt Tools”.
4) Click the “Rotate à Flip Horizontal” option under “Arrange” section.
Check the following link for further details.
http://office.microsoft.com/en-us/word/HA102403771033.aspx