People commonly ask about using a column of data type "Person or Group" in another column of data type "Calculated".
Bottom line, this does not work in WSS 3.0 (or MOSS).
When adding a calculated column, WSS shows the list of fields it allows you to use for the calculation. If you type in the name of a column that is not in its list, it tells you:
One or more column references are not allowed, because the columns are defined as a data type that is not supported in formulas.
Workaround: Use an event handler. The event handler fires when the user saves the item. It performs the update you would have wanted the calculated column to do for you.
Useful links on calculated fields in general:
Quick disclaimer: I believe the above to be true and accurate, but I’ve seen enough clever tricks here and there in MOSS/WSS that I wouldn’t be extremely surprised (agog if you will) if someone has figured out a way to do this without resorting to code. If you’ve figured out clever work-around or know someone that did, please let me know!