One would like to be able to use dynamic information like [Today] or [Me] in calculated columns (for example, to compute "Age" from "DateOfBirth"), but SharePoint complains.
Steve Eagleson was kind enough to point out this article from "Novotronix":
Novotronix Blog - Using [Today] or [Me] in SharePoint calculated columns
which contains a workaround which seems to... um, work.
The workaround involves creating a temporary column called "Today", then creating the formula, then deleting the temp column. This suggests that SharePoint actually knows how to use the dynamic variable, but the parser rejects it when the formula is saved. Very odd...