keyboard shortcut
That doesn't really answer the question.... which was for a KEYBOARD shortcut. I just started using Numbers instead of Excel myself and I am finding the lack of keyboard support particularly annoying. Especially the lack of a shortcut for Fill. Fortunately you can assign your own shortcuts in Mac OS X
Goto Systems Preferences, select Keyboard, and Click on Keyboard Shortcuts, Click the plus "+" sign to add a shortcut, select or Add Numbers as the Application and put "Fill Down" in the command spot and then assign your shortcut. I assigned command D, similar to Excel.
Now I must rant about the lack of keyboard support in Numbers. I really don't understand why Apple didn't include shortcuts like the one above by default. It's kind of an obvious thing and any Excel power user would be quite unhappy without it. There are also some other annoyances with the keyboard such as the inability to specify a range with the cursor keys when entering a formula. For example Try typing "=sum(" and the using your cursor and shift keys to specify the range... you can't do it, you have to move your hand all the way over to your mouse, position, click and drag for the range... This takes way longer to do, especially if you're just doing a quick little sum or other function where the cells are very close by. While the mouse can be a very useful accessory I really dislike being forced to use it unnecessarily. The mouse should be used to help us not hinder. The invention of the mouse was a great thing but it is not the only way to do things... I would be willing to wager large amounts of money that a Excel power user that knows all the keyboard shortcuts is an order of magnitude faster than someone who uses the mouse. If Apple wants those same power users to switch to Mac/Numbers they should have the same capabilities in their software.