Deep within the bowels of the Microsoft on-line help can be found:
which lists the derivation of trigonometric functions not provided by VBA. Not included in this list is ATan2(), which is strange because this is the one trigonometric function that is really essential for anyone doing trigonometric or vector based calculations.
The file: http://www.interactiveds.com.au/software/TrigFuncsVBA.zip provides UDF versions of all the functions listed on the Microsoft site, plus 2 versions of ATan2().
See the next post for an illustration of the advantage of using a UDF for Excel functions that are not included in VBA, rather than using the WorksheetFunction object.