Is the iPhone 4 antenna problem fixed in the iPhone 4S
I think so. Since Apple placed an extra antenna (now they are two) in the iPhone and they redesigned the whole antenna system (an extra receiver in the frame inter alia). 



And according to the movies in this blogpost, it seems like the new system did fix the so-called antennagate! They compared the iPhone 4S reception with an iPhone 4 which is suffering from the antennagate.

According to Apple itself, the system...


  Intelligently switches between two antennas to transmit and receive