I think the safe answer is that it shouldn't be a problem. but TSA can do whatever they want apparently. Had a friend travelling through JFK last week and they took his Microsoft Band - for those that aren't aware of what it is, it's pretty much like a fitbit, but with some smart watch capabilities (notifications, calendar reminders..etc). Some how I doubt they are confiscating everyone's Apple Watch... So you never know..
We have taken a radio before though, wasn't an issue.
On return through San Juan though - what a mess at security. So many people had stuff in their bags they shouldn't have that there was a huge delay for anyone that had stuff that was ok, but needed to be checked.. In our case, we had two bags held as one had water (we were travelling with our 6mo old son) and the other had diaper rash cream. Both were ok, but we had to wait about 15-20 minutes because so many people had stuff they were not allowed to take! <img src="http://www.traveltalkonline.com/forums/images/graemlins/duh.gif" alt="" /> Always pays to leave extra time. Funny thing about it was there was this couple that we saw as we entered the line and they were anxious because they were running late - well... they had something in their bag they shouldn't have and delayed themselves even more! <img src="http://www.traveltalkonline.com/forums/images/graemlins/jester.gif" alt="" />