Justice Kennedy, who wrote the opinion found that, "Because the search of Quon's text messages was reasonable, petitioners did not violate respondents' Fourth Amendment rights, and the Ninth Circuit erred by concluding otherwise."
The ruling was issued on very narrow grounds as the Court was concerned that there was a risk of unintended consequences if it made broad new legal rules on a case involving new technology, though the case itself involved nearly obsolete two-way pagers. Nevertheless, the Court has demonstrated it is clearly hesitant to create rules regarding technology before the technolgy is fully understood. Justice Kennedy, stated that doing so "might have implications for future cases that cannot be predicted."
So what does this mean for you, if you work as a public employee?
Basically, don't send anything from a government-owned device or through a government owned server if you want to maintain your privacy. The 4th Amendment's protections against unreasonable search and seizure still apply to your personal cell phone and your home computer. So if you insist on sexting, don't do it from your government issued device.