Alt+M.
(it really works, the text of the 'Merge and Center' button on the excel toolbar must be showing)
excerpted from http://www.1-on-1.biz/MicrosoftOffice/MicrosoftExcel/mergeandcentershortcut.aspThe shortcut for Merge and Centre is in fact [Alt] + [M] however it does not actually work when only the Merge and Centre shortcut button is shown on the screen. 
When both the Image and Text are showing you can actually then use the Merge and Centre Shortcut [Alt] + [M] to merge and centre your Microsoft Excel Cells.
To customise your Formatting Toolbar so that you can see both the Image and the text of the Merge and Center Shortcut, first choose the Tools menu and then choose Customize from the drop down menu. In the Customize dialog box choose the Commands tab.
Now select the Rearrange commands button below the Categories and Commands List box.
In the Rearrange Commands dialog box, first choose from the Toolbar drop down box the Formatting toolbar option and then in the Controls list box Merge and Center.
The next step is to choose the Modify Selection button on the right hand side and the shortcut shown will be visible. The Default Style option will be chosen but you want to choose Image and Text. Once you have done that, press the Close button and then press Close for a second time.
If you now select a group of cells in your spreadsheet and go [Alt] + [M] you will then be able to Merge and Centre using a Merge and Center shortcut keystroke.
The next step is to choose the Modify Selection button on the right hand side and the shortcut shown will be visible. The Default Style option will be chosen but you want to choose Image and Text. Once you have done that, press the Close button and then press Close for a second time.
If you now select a group of cells in your spreadsheet and go [Alt] + [M] you will then be able to Merge and Centre using a Merge and Center shortcut keystroke.
