dates
January 1 2000 (no commas)
It is occasionally alleged that putting month before date in this way is an "Americanisation"; in which case it should be pointed out that this has been our style since the first issue of the Manchester Guardian on May 5 1821

http://www.guardian.co.uk/styleguide/page/0,,184835,00.html

Comma if month is first, no comma if day is first.

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Note added at 2 hrs (2006-05-08 02:30:07 GMT)
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MLA Handbook for Writers of Research Papers, p. 49, section 2.2l:

Use a comma in a date whose order is month, day, and year. If such a date comes in the middle of a sentence, include a comma after the year. "Martin Luther King, Jr., was born on January 15, 1929, and died on April 4, 1968."

But commas are not used with dates whose order is day, month, and year. "Martin Luther, King, Jr., was born on 15 January 1929 and died on 4 April 1968."

As we can see from two previous answerers, citing style guides, this is an area where individual publishers have their own style guide.