If you do a search in the "customizing arcpad" help for "Project Example", it'll be the eigth one down the list. It's basically an example script on how to project from one coordinate system to another, which is a pretty good place to start.
However, if your map is in the coordinate system whose easting and northing you want to retrieve, you could just pull those values from the feature itself like this:
Dim objSH
Set objSH = Application.Map.Editlayer.Records.Fields.Shape 'Sets the objSH variable to the currently being edited feature's "shape" which contains the X / Y coords.
EditForm.Pages("thePage").Controls("theEastingComboBox").value = objSH.X
EditForm.Pages("thePage").Controls("theNorthingComboBox").value = objSH.Y
You could put the above (modified to suit your project of course) in the onload event of your editform I think. If that doesn't work (which might be the case because the feature hasn't been saved yet, and therefore has no "shape" to reference until OK is hit, I don't remember), then try and just use Map.PointerX and Map.PointerY to get a reference to the current location. They'll be set to the current feature's location if you just used the "create point at GPS location" button.
Hope this helps,
Eric
EDIT: I have no idea how this would work for features that get thier position via averaging or post-processing. Might be easier to just calculate them after the fact in ArcMap unless the field crew really needs to see the exact values.
