Well, when I got the new phone it was different. It was different hardware. The black [plastic] bezel isn't as black on the new one. I couldn't see the proximity sensor at all on the previous iPhone 4, now I can. The stainless steel band on the new phone is less 'steel-y' and more matte. I've also tried to replicate the signal drop and failure. While I can't say for sure that it is entirely fixed, there is certainly huge improvement. I'm guessing they coated the steel with somethingThe report had raised some eyebrows but without further evidence, it was hard to put much faith into it alone. However, iFixit also received a single report from one of their readers of a similarly updated iPhone 4.
iFixit even ran out and even bought another iPhone 4 to check, but the new iPhone wasn't built much later than their original (week 27 vs week 26). As a point of reference the conductivity of their original iPhone 4 was measured as such:
Original iPhone, high conductivity
Lower numbers indicate higher conductivity. If Apple truly did add a non-conductive coating to the iPhone 4's antenna, iFixit suggests the numbers should be in the thousands. We did manage to track down a week 28 (manufactured this week) iPhone 4 and found no difference in conductivity compared to the original iFixit measurements.