Previewed at WWDC, launching in the fall.
iPhone 6 May Include Temperature, Pressure and Humidity Sensors
G for Games relayed the report and points out that the "pressure" in this context is certainly atmospheric, not blood pressure:
According to Sun Chang Xu (news chief analyst at ESM-China), sources close to the matter have revealed that Apple will catch up in the “sensors department”, as the iPhone 6 will feature pressure, temperature and humidity sensors. Please note that the analyst is probably not referring to blood pressure (this feature is rumored to be implemented in Apple’s iWatch) but to atmospheric pressure.This same analyst previously reported that Apple may use "optical sensors" to measure heart rate and oxygen levels in the rumored iWatch. The addition of these atmospheric sensors would bring it up in line with Samsung's S4 which included similar atmospheric sensors.
The addition of more sensors to the iPhone 6 seems a natural progression with all the recent reports that Apple has been aggressively hiring individuals in the area of health sensors. iOS 8 is said to include a Healthbook app which reports on many health related sensors.