In order to be able to utilize services from ArcGIS Server in ArcPad, you would need the ArcGIS Server ArcPad Extension. However, that extension was first released for ArcPad 8. Therefore, if you are using ArcGIS Server 9.3, you will require the ArcGIS Server ArcPad 8 Extension. If you are using ArcGIS Server 10, you will require the ArcGIS Server ArcPad 10 Extension.
So in order to be able to consume services in ArcPad and because you are running ArcGIS Server 10, you would need the ArcGIS Server ArcPad 10 Extension. Therefore ArcPad 7.1 would need to be upgraded to version 10.