I don't use Numbers but in Excel, a suitable formula would be:
=[Initial]-INT((TODAY()-DATE([Year], [Month], [Day]))/7)
Where [Initial] is the starting value and [Year], [Month] and [Day] refer to the day on which you want the countdown to start. Hence, a cell that started at 100 and reduced by 1 every 7 days from today would be generated by:
=100-INT((TODAY()-DATE(2009, 5, 27))/7)
I assume Numbers has equivalent functions. INT() is the integer function - it returns the portion of the expression in the brackets that is a whole number and disregards the rest (i.e. the decimal value) - otherwise the initial number would decrease by 1/7 each day. TODAY() returns today's date and DATE() returns the date defined by the terms in the brackets.
Hope that helps!