If you're running Office 2003 (I don't know if this works for 2007), you can set it up so that Alt+m will merge your selected cells:
1. Choose 'Tools'
2. Choose 'Customize'
3. Right click on your merge and centre icon on the toolbar at the top of your screen
4. Click 'Image and Text'
5. Close the Customize window
The shortcut Alt+m should now work.
It seems silly that this works, but it's a godsend for me.