I think I have solved my own problem here. I'll post my solution so anyhow wondering in the future about the same issue might use this.
The typelibrary as generated by Delphi on import of the COM object generates the type PhidgetSpatial_SpatialEventData which I expected to get in the OnSpatialData event. However this event has a parameter 'data' of type OleVariant. I can access the data in the OnSpatialData event by adding the line
PPhidgetSpatial_SpatialEventData = ^PhidgetSpatial_SpatialEventData;
in the generated typelibrary.
In the eventhandler I can access the data by doing this:
procedure TFormMain.PhidgetSpatialSpatialData(ASender: TObject;
var data: OleVariant; dataCount: Integer);
PData := PPhidgetSpatial_SpatialEventData(VarArrayLock(data));
Label1.Caption := Format('%g', [pData^.Acceleration]);
Label2.Caption:= Format('%d', [pData^.time_seconds]);
As far as I can see this gets me the correct values. I came up with this solution by looking at the provided VB sample and googling for OleVariant handling in Delphi.
Hope this will help others in the future.