Does anyone track purchase order spend against the PO amount in Apptio?  

Not yet. BUT today we kick off the Vendor Insights module work which will include PO tracking and many other highly anticipated features. Short of world hunger, this module will solve many of our ccurrent challenges.

We do in our Vendor Insights module.  We map accounts payable invoices to the purchase orders and report against this.  It has helped us in the control of budget held against unused POs.