HI homeguard,
You can do this by number of ways, one of my logic i thinking is take a form with PO field and submit it in share point.
From your form take the secondary connection of that form that contained PO and take the value of PO to the present form and increment by 1 in the secondary dom and submit the both forms at a time. So that while taking the secondary form that having PO will increment by 1 in the next time.
Like wise you can do in number of ways.
Hope this will help you.
B.Shiva Prasad
InfoPath Dev,
http://www.ggktech.com
India