public void ZoomToSelectedFeatures()
        {
            ESRI.ArcGIS.Carto.IActiveView pActiveView;
            ESRI.ArcGIS.Carto.IMap pMap;
            ESRI.ArcGIS.Geodatabase.IEnumFeature pEnumFeature;
            ESRI.ArcGIS.Geodatabase.IFeature pFeature;

            pMap = (IMap)m_hookHelper.FocusMap;
            pEnumFeature = (IEnumFeature)pMap.FeatureSelection;
            pFeature = pEnumFeature.Next();

            ESRI.ArcGIS.Geometry.IEnvelope pEnvelope;
            pEnvelope = new EnvelopeClass();

            while (pFeature != null)
            {
                pEnvelope.Union(pFeature.Shape.Envelope);
                pFeature = pEnumFeature.Next();
            }

            pEnvelope.Expand(1.2, 1.2, true);
            pActiveView = m_hookHelper.ActiveView;
            pActiveView.Extent = pEnvelope;
            pActiveView.Refresh();
        }

http://forums.esri.com/Thread.asp?c=159&f=1707&t=233785

Here is the sample code you can use to zoom to selected features. Please replace m_hookHelper with valid mapcontrol object.

Zoom to selected feature(s)

C#