Help with sumproduct or similar type of function

[jasondebolt]

I am interested in finding a function that will count the number of
unique combinations a in two columns (columns A& B). For example:

Column A Column B Column C Column D
ABC 03/2007 =A1&B1 =SUMPRODUCT((1/
COUNTIF(C1:C5,C1:C5&"")))
BCA 03/2007 =A2&B2
CAB 07/2007 =A3&B3
ACB 07/2007 =A4&B4
ABC 03/2007 =A5&B5

In the case above, there are 4 unique combinations in columns A and B
(ABC 03/2007, BCA 03/2007, CAB 07/2007, ACB 07/2007). I don't count
the second instance of ABC 03/2007 becuase it is already in the first
row.

Is there a cleaner way of doing this? I've tried using sumproduct, but
I can't seem to get it to work the way I want it to. Does anyone know
how this can be done?

There are only four possible strings in column A (ABC, BCA, CAB, ACB)

Thanks!
Jason