Ubisoft announced yesterday that Far Cry 3 will be delayed to later this Fall/early Winter in the Europe and US. However, they said nothing about the game's beta period that is supposed to start this summer.
Since Ubisoft has since explained that they are holding off on releasing the game due to quality concerns, then it's easy to conclude that the beta will also be postponed for quality assurance reasons. However, Ubisoft has admitted that they have no idea how this will affect the beta. In fact, they told Gamespot that they have made "no decisions" in regards to the beta's time frame.
If you've signed up for the beta, I guess you can be rest assured that you know as much as Ubisoft does.