Has the design-flaw with the Boeing 737's cabin-pressure switch been fixed yet?
The Wikipedia article on Helios Airways Flight 522 also mentions this wiring issue, and cites a separate report regarding a May 2003 incident in which a 737 had issues with pressurization. During the investigation it was discovered that wiring had shorted and burned in a particular bundle of wires, called a loom, which was important for control of pressurization. The AAIB issued a safety recommendation regarding this issue:


  Safety Recommendation 2004-33
  
  It is recommended that in order to prevent failure of the cabin pressure control system in the event of damage to wiring loom W298, the Boeing Commercial Airplanes should consider, on the Boeing 737-436 and similarly configured models, separating or protecting the wiring associated with the different modes of operation of this system, which connects the cabin pressure controller to the rear outflow valve, such that any single point failure of the loom would not result in effective failure of the pressurisation control system.


In their 2006 progress report, the AAIB followed up on this recommendation:


  After a review of the issue and the service history of the pressurization system for the 737-100 thru - 500 model airplane, Boeing felt that modifications of the wiring with the intent of separating the outflow valve functions consistent with the subject safety recommendation would not significantly contribute towards a reduction in the overall rate of depressurization events. Their rationale was based on the basic design philosophy and concept of the original system which includes:
  
  1) A single, mechanical outflow valve which is controlled by a single multi-mode analog pressure controller and driven by independent AC or DC powered electrical motors. Each motor can also be manually commanded open and closed via separate switch on the pressure control panel.
  
  2) In the event of a complete power/control loss (e.g. - electrical interruption or damaged wiring) either motor will remain in last commanded position. The airplane is equipped with an independent aural warning system (cabin above 10,000 feet) and visual cues (cabin above 14,000 feet) should this condition lead to a cabin pressure loss.
  
  3) The airplane is equipped with both fixed and portable oxygen supply systems for passengers and
  crew.
  
  4) If pressure cannot be reliably controlled/restored within a short period of time, the flight crew will utilize procedures (such as rapid reduction in altitude, declaration of emergency and landing at nearest suitable airport) which mitigate the exposure time to low pressure.
  
  Status - Rejected - closed


So the rationale is that if the system fails, the crew is alerted and may take appropriate action. Note the following regarding an AD published in 2011 with a deadline of March 2014:


  The FAA released Airworthiness Directive AD-2011-03-14 requiring all Boeing 737-100, -200, -300, -400 and -500 types to install two additional warning lights, one for takeoff configuration and one for cabin altitude warning, and introduce updated documentation within 36 months following March 14th 2011.