See solution in other versions of Excel:
Question: I want to center text across multiple cells in Microsoft Excel 2003/XP/2000/97 but I don't want to have to merge the cells. How can I do this?
Answer: Select the cells that you wish to center the text across.
Right-click and then select "Format Cells" from the popup menu.
When the Format Cells window appears, select the Alignment tab. Click on "Center Across Selection" in the drop-down box called Horizontal.
Now when you return to your spreadsheet, you should see the text centered across the cells that you selected.