WIPO has decided to let third parties submit observations electronically through Patentscope (reg). These must be filed within 28 months of the priority date. For an overview of the PCT have a look here.
The observations will be limited to novelty and inventive step.
I think that this is a great way to head off potential problems. The PCT is the gateway to multiple foreign filings. Jamming up a competitor at an early stage could save a fortune in litigation fees. I recommend a regular check on published PCT applications for those rolling out or developing technologies across multiple jurisdictions.
Timing is an issue. PCT applications are published about 18 months after the priority date. So there's about a 10 month window to submit the observations.