Assuming you are not trying to call the DateDiff function in the IIF statement, while you can combine it into one statement:
=IIf(IsNull([Bleh]),IIf([fldDateDiff]<7,"",IIf([fldDateDiff]<14,"7 Day Warning",IIf([fldDateDiff]<27,"14 Day Warning","28 Day Warning"))))
I think a better solution is...