Do swim caps make a difference?

No and no. 

It depends on what you mean by make a difference,

to speed - depends on what/where/howlong you are swimming.

to heat loss - yes it makes a big difference.

some pools requires long-hair people to wear a cap.  Even with my short hair (short for a girl), if no swim cap, some of the hair gets in my face and block vision/breathing. 

OWS - good to wear a bright color cap so that your buddies can see you. IN colder water I wear 2 caps to keep warm. 

During a race - cap is mandatory.  Caps are part of the race packet along with your number and timing chip. 

Mark Spitz won 7 gold medals with no cap, no goggles, a mustache, and a regular old speedo (I think they were just nylon or polyester, no lycra back then).  So yes, you can be pretty fast without all the stuff.

About half the guys I swim with wear caps in practice.  Mainly for protecting their hair, I assume.  I wear one in the pool only when the water is colder.   I always wear a bright one in open water just to make myself a bit more visible.

I'm a relatively good swimmer by triathlon standards, but I doubt that the cap makes me measurably faster...

Brian

I just started wearing one for this same reason.  

It dawned on me a couple weeks ago, that I don't like wearing swim caps.  It was uncomfortable to me.  So, why would allow myself to be uncomfortable at the start of every race?  Now, I wear one in the pool for training.  My hair seems to collect less chlorine too.  I'm not sure if that is true or not. 

Same for me.  HATED wearing my swim cap for my first tri in July.  I knew I would need to wear it in future races, so I wear it in training now.  I have gotten used to it.  The Tyr one I got on race day is comfy, but the one I got from Xterra, free, with my wetsuit doesn't work for me.  I have a latex allergy, and the Xterra cap aggravates that.  Nothing like burning eyes/forehead to make the day better.    

1.  It's warmer when I swim outdoors in cooler weather.

2.  It "feels" faster to me, even though it probably is just psychological

Mark

I've always worn one since I've got into tris.  More for being used to it and keeping hair out of my face.  No other reason really.

To me it feels weird NOT to wear one!

Yes, of course they make a difference.  They let everybody at the pool know that you are a serious swimmer, that even during your workouts, you absolutely must shave that extra .01s from your time.  And if you have one of those red white and blue ones, it also indicates that you are patriotic and they are not.

OK, well, I wear one.  I agree that it feels better.  On race day, I had enough other things to deal with -- adding getting used a swim cap might have pushed me over the edge.