Chris wrote an answer about MSR Fuel Pump on July 12, 2011
I've used a WL for years. I think the problem you're referring to is a flaw in this new design: The valve control is now that long affair that folds down, instead of the old thumbscrew. As a result, when it's folded, it acts as a lever arm and normal bumping around in a pack can cause the valve to open slightly. Bad news! Especially because there is no fix available. The best you can do is be sure to close it tightly, and then pack it so that the valve is reasonably protected.
Everything else about the new design is good. This control arm thing, however, was a mistake.