OpenGL is a suitable choice for a graphics API. There's a fair bit of legacy cruft floating around in the API, especially if you are not using the latest version, which can be distracting and confusing for a newbie. It's also a fairly low-level system and it's possible you may want to investigate other potential 2D engines that provide a higher-level API. This is especially true if you only want to focus on 2D graphics initially, because OpenGL is a 3D API -- it has no "2D mode," and if you don't understand the fundamentals of the 3D graphics pipeline that you're using, even when faking 2D with it, you can get yourself into some uncomfortable spots with weird, difficult-to-diagnose (for you) bugs.
There's a lot of questions on the subject of 2D engines here.
OpenGL, Direct3D, whatever -- it's more important that you learn the fundamentals of graphics programming. That skillset will transfer to any API or any engine, which means when changing platforms you simply need to learn another API and how that API exposes those fundamental, shared concepts, which is trivial. Or at least should be, for a good programmer -- if it is difficult for you, you just need to practice it more.
Does OpenGL offer a free/low cost to
entry as I think it does?
Yes, it costs nothing to use OpenGL itself.
How well does the skillset Xfer to the
iPhone (I believe I'll need to use C
on the PC and Obj-C on iPhone)?
You can use C and C++ on iOS as well -- you'll require a minimum amount of Objective-C bootstrap, but the bulk of your program can easily be written in other languages. The iPhone provides you the OpenGL ES API, which is essentially a subset of the full OpenGL API. It should be relatively easy for you to adapt your code to run on an iOS device.
Finally, what are the "big-3" using
for development (PS3, Nintendo, Xbox)?
It doesn't matter. This kind of question is indicative, usually, of a concern that you should pick the one "right thing" to learn and focus on that. But there isn't a right thing and if you know the basics, as I noted in the initial part of this answer, you can learn any API presenting those basics with relative ease.
That said, the PS3 has an OpenGL implementation that nobody takes seriously and a proprietary API for anything real. The 360 uses an API that is quite similar to Direct3D. The Wii probably uses something OpenGL-like as well, although I couldn't say for sure.
On "Your Needs:"
We can't really answer that for you, since we only have the most cursory understanding of your needs and your ability. Just like learning APIs, evaluating requirements and potential solutions for those requirements is an important part of a developer's training.
The best way to do this is to try it -- set up a project and try to build a quick tech demo with OpenGL. You'll get a feel for how the API works, where you can find good reference material (because you will invariably get stuck somewhere and have to hit up Google to find out, for example, "how to create a shader in OpenGL."
You can then grab some higher-level 2D engine and try to recreate the same tech demo and see how you feel about the relative experience. You can try to demonstrate features you know you'll want in your game -- if you want fancy alpha-blended explosions, try building demos that render a bunch of transparent sprites (or even just blank, colored quads) intermixed with some opaque ones. You'll discover the problems inherent in rendering transparent graphics and you'll have to solve them with both pieces of technology you're evaluating. One will, perforce, work out better/easier for you. That may be the API you want to actually stick with.