AlertDialog.Builder alertDialog = new AlertDialog.Builder(this);
			alertDialog.setMessage("有新消息，是否查看？");
			alertDialog.setPositiveButton("否",
					new DialogInterface.OnClickListener()
					{
						public void onClick(DialogInterface dialog, int which)
						{
						}
					});
	
			alertDialog.setNegativeButton("是",
					new DialogInterface.OnClickListener()
					{
						public void onClick(DialogInterface dialog, int which)
						{
						}
					});
	
			ad = alertDialog.create();
			
	        ad.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);  
	        ad.setCanceledOnTouchOutside(false);//点击外面区域不会让dialog消失                               
	        ad.show();

<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />