How to count network days between two dates, using todays date where second date is blank?

Hi

I am looking to create a formula that will count the number of work days (network days) between two dates and to use today's date where the second is blank.

The intention is that I can show how old incidents are in the one cell, regardless of whether they are open or closed

Here's the layout for what i want to achieve:

Col A, Col B, Col C
Row 1, Open Date, Closed Date, No.Workdays Open
Row 2, 05-Mar-2012, 12-Mar-2012, 6
Row 3, 16-Mar-2012 , *, 2

*where today is the 19 March


So far I have worked out how to use Network Days
=NETWORKDAYS(A2,B2)

And how to use today's date where col B is blank
=IF(ISBLANK(B2),TODAY()-A2,B2-A2)

But I can't seem to combine the two.

Any suggestions greatly appreciated.