[SalesForce] Formula to display the current week

There are Date Literals you can use in the query criteria. I am not sure if you are looking for last calendar week or last 7 days week, but you are covered either way plus several others-

[SELECT Id, Name FROM Opportunity WHERE CloseDate = LAST_WEEK]

Starts 12:00:00 on the first day of the week before the most recent first day of the week and continues for seven full days. First day of the week is determined by your locale.

or

[SELECT Id, Name FROM Opportunity WHERE CloseDate = LAST_N_DAYS:7]

For the number n provided, starts 12:00:00 of the current day and continues for the last n days.

Nothing simple exists that I'm aware of.

However, check out the useful advanced formulae page: http://login.salesforce.com/help/doc/en/useful_advanced_formulas.htm

In particular, this has a formula for calculating the day of the week as a number (0 = Sunday...6=Sunday).

MOD(entry_date__c - DATE(1900, 1, 7), 7)

The above will give you Entry Date's day of the week. To get Sunday, you should be able to subtract the result of that from the Entry Date

DATE(
    YEAR(Entry_Date__c),
    Month(Entry_Date__c),
    DAY(Entry_Date__c)-MOD(entry_date__c - DATE(1900, 1, 7), 7))

Though you would need to add some complexity to this to handle the start of a month/year.

Edit:As @TomLogic points out below, you may be able to avoid using the DATE function and having to add complexity to handle start of month/year issues by subtracting the result of the Mod operation from the Date directly (though I've not yet tested it!)

Entry_Date__c - MOD(entry_date__c - DATE(1900, 1, 7), 7))