And this could be the true Excel killer. If we figure out a paradigm for nested structures (or simple sql relationships really) that is as easy to work with as Excel we would have a very strong case for moving users away from spreadsheets. Every time data is interrelated in Excel it becomes a hot, very manual, mess.
Excel allows creating primary/foreign key relationships between tables using the Data tab, but you can only use these relationships to produce pivot tables, not in normal formulas.