Al has higher conductivity than SS. I'd have to crunch numbers to see if it would make that much difference at cell phone frequencies (power budget). At 60 Hz, ACSR has most of the current in the Al not the steel from the skin effect.

SS has the advantage of strength over Al.

Possibly a SS exterior with Al inside.

edit: I cheated and glanced at the ARRL Handbook (2005) page 20.7 "The strongest wire suitable for antenna service is copperclad steel, also known as copperweld. The copper coating is necessary for RF service because steel is a relatively poor conductor."

Based on that I suspect a better conductor inside the SS or as part of the SS.

Further edit: using google, there are SS Yagi antennas so that is nothing new, not sure about other antenna types at this time. If Apple used SS and made a better antenna (larger?) then it might use less power than a smaller antenna using a better conductor.

last edit: SS whip antennas are also available, such that the material choice probably is not that big of deal as much as the design of the antenna itself.