In Numbers 3.0, if I type in December 10, 2013 it stays formatted that way. And that is true for any other way I type in a date (as long as it is an acceptable format); they stay just as I typed them.
In Numbers 2.3 (Numbers '09), though, the default format is Dec 10, 2013 no matter how I enter the date. The only way to get around it is to pre-format the cell(s). If you are often changing formats as you enter dates and you want them to display as typed, you will often be going to the cell tab of the inspector and changing the format. But this is not typical. Typically you want all the dates formatted the same way so pre-formatting the cells is a one-time action.