There isn't by default, but you can assign one. Go to System Preferences -> Keyboard -> Keyboard Shortcuts. Choose "Application Shortcuts" from the list on the left, click the plus (+) button at the bottom of the list, choose Numbers from the drop-down menu. For Menu Title, enter "Merge Cells" (without the quotes, of course), and then pick whatever keyboard combo you want. Hit OK, and go back to Numbers and it will be working immediately, no need to relaunch.
jW