Help for LINEST states that

The array known_x's can include one or more sets of variables. If only one
variable is used, known_y's and known_x's can be ranges of any shape, as long
as they have equal dimensions. If more than one variable is used, known_y's
must be a vector (that is, a range with a height of one row or a width of one
column).

In particular,
=LINEST({1,1.5;2.5,3},{1,2;2,3},,TRUE)
should be equivalent to either
=LINEST({1;1.5;2.5;3},{1;2;2;3},,TRUE)
or
=LINEST({1,1.5,2.5,3},{1,2,2,3},,TRUE)

Prior to 2003, they were equivalent (as documented), but in Excel 2003, not
even the degrees of freedom for =LINEST({1,1.5;2.5,3},{1,2;2,3},,TRUE) match
the documented behavior.

What happens in 2007?

Jerry