If a column is null, does that affect the space used by the column?
Is the space used fixed by the column definition?
Does this vary from database to database.
(I am mainly interestred in SQL Server 2000.)

Clarification:
The question relates not to what happens when the column is 'nullable' (This costs another bit as Kritsen & gbn pointed out). The question is, is there any saving when the column is actually null (in some particular row). 

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Cadaeic provided the answer for SQL Server which seems to be no savings in SQL Server until version 2008, whereas according to Quassnoi you can get savings in Oracle if the null columns are at the end. Thanks for the answers, they were all helpful.