Just to add to Shane's response...
You can format the cell to show both the date and time and it'll update as often
as excel recalculates.
But if you use it to compare dates like:
=if(a1=date(2009,12,25), ... )
or
=if(A1=b1, ...)
where A1 contains =now() and B1 contains a date (without the time), then you may
not want to include the time by using =now().