I have an iPhone charger (produces 5V and 1A output). Is it okay to use this to charge my Nexus S?
As long as it is standard micro USB charger, it should be fine. Per spec, dedicated USB charging ports can supply up to 1.5 A at 5 V.
Check the battery label to see the rated maximum charging current to be extra safe, but it should work properly. USB devices are very common and standardized these days, and USB chargers work without a hitch across multiple devices.
Then again, Apple has been known to use not-quite-standard ports (I'm looking at you, 3.5 mm audio jack!), so take this with a grain of salt.