<p>It seems that all of the documentation I can find about OpenGL-ES says something to the effect of "OpenGL-ES is just like OpenGL, but without a lot of stuff.  For example, there's no glBegin or glEnd."</p>

<p>Ok, that's great.  So, what ELSE isn't there any of?  Or is there a list of what's <em>in</em>?  Or maybe a porting guide?</p>

<p>(Specifically, I'm trying to move an existing GL app to the iPhone, although I don't want to necessarily limit my Q to the iPhone.)</p>

The
"OpenGL ES 1.1.12 Difference Specification"
(PDF) linked to from
the OpenGL ES 1.X info page at Khronos.org
goes through the differences between OpenGL ES 1.X and OpenGL 1.5.  OpenGL ES 1.1 is the version used on the iPhone.
The difference specification is not the simplest document I've ever seen, but it is easier reading than the OpenGL specs in general.  I recommend getting a list of OpenGL functions you call and then searching through the difference document for them.  It will show you if they are supported in OpenGL ES, and if support is only partial you can go to the full spec for more information.