F-35B
Lockheed Martin
The F-35B (click to expand)
The F-35 is unable to fly within 25 miles of a thunderstorm because engineers believe it could explode if struck by lightning.
The storm restriction will not be lifted until an oxygen gauge in the fuel tank is redesigned in all F-35s.
The F-35 Joint Strike Fighter (JSF) program has experienced setbacks since 2000 when the first concept designs by Boeing, and winning contractor Lockheed Martin, were scaled back due to cost overruns and development delays. Current estimates place the cost of operating the projected U.S. fleet of F-35s at about $1 trillion over the five decades it's expected to serve the Navy, Air Force, and Marines.
This most recent development is mentioned in the Pentagon's 2012 Operational Test and Evaluation report that annually examines developing defense programs until they reach full production.
The section on the F-35 doesn't mention the critical fuel problem until the second page where it seems to have been known about since 2009.
From the DoD report:
Tests of the fuel tank inerting system in 2009 identified deficiencies in maintaining the required lower fuel tank oxygen levels to prevent fuel tank explosions. The system is not able to maintain fuel tank inerting through some critical portions of a simulated mission profile. The program is redesigning the On-Board Inert Gas Generating System (OBIGGS) to provide the required levels of protection from threat and from fuel tank explosions induced by lightning.
Lockheed released a January 11 F-35 program highlight sheet mentioning the F-35B accomplished 396 flights, met more than 2,400 test points, and executed 102 vertical landings. Of the 30 JSF deliveries made in 2012 only one was an F-35B and that was made to Britain's Royal Air Force in October. The F-35B was grounded late last week diue to problems with its fueldralic line.
Part of Lockheed's, and the Pentagon's success at signing countries onto the program is integrating its development into foreign economies, making jobs a part of final delivery. The Telegraph points out that concern in an article today:
The future of the aircraft is also key to Britain’s defence industry and will help to sustain over 20,000 jobs.
Although the plane is being manufactured by Lockheed Martin, Britain is a major partner in the programme, with both BAE Systems and Rolls-Royce playing key roles in the production and design of the jet.
The UK already cut its F-35B order to 48 from the 2006 order of 138 jets.
A spokesman for Lockheed Martin told The Telegraph: "“The F-35 is a stealth aircraft and by definition it is less vulnerable than any fourth generation fighter flying today. We don’t consider this a major issue. We have demonstrated very good vulnerability performance and we continue to work this with the Joint Programme Office.”
All of this comes after Canada's December announcement that it would be stepping back from its F-35 commitment. 
Australia followed Canada's announcement a few days later saying it would buy 24 Boeing F/A-18 Super Hornets if it saw any more delays in the F-35 program.
Firming its support for the program the Pentagon promptly announced it was signing a contract with Lockheed Martin for a fifth batch of 32 jets worth $3.8 billion.
U.S. pilots started training in the F-35 at Eglin, AFB starting this month can likely plan on duty stations at Marine Corps Air Station Iwakuni, which will be the first U.S. F-35 overseas base in the world. 
All of that is good news for Lockheed Martin and its investors who now expect profits in the high single digits following about 10 years of four percent profits during the F-35's development phase.