Here are two easy options that I am a fan of:
(for Excel 2003... I don't know if these work on Excel ≥2007)


Quickest and easiest is just to perform the command normally one time, and after that just press F4 to 'repeat action'.


Another option is:
1. Right-click the 'merge and center' toolbar icon at the top of the screen, select 'Customize'
2. Right-click the 'merge and center' toolbar icon again and select 'Image and Text'
3. Close the Customize window

Now Alt+m should work as a key command every time (note the underlined 'M' in the merge and center button). I can't believe this only works with the text displayed on the button (especially since Alt+m does nothing anyway!). It's a bit annoying if you're like most people and only have the icons displayed on the rest of the toolbar—of course, you could revert back to the 'default style' (icon only) when finished.