The interesting and powerful things about Smalltalk are not found in the bits, but in the ideas (it took me a few years to realize this).
Without this realization, what will probably happen is that you'll port the ways of thinking from whatever your first programming language is to Smalltalk syntax (I'm still brain damaged from C after 15 years), missing the real juice (although the tools, closures, live system, etc. will definitely lead to productivity).
What most helped me rearrange my neurons to realize what matters about Smalltalk were:
That being said, I've been very pleased with Pharo for learning, researching, and production development. I think it's perfect for learning because ugly code is rapidly disappearing in favor of clean, well thought out, fun to use replacements. Check out FileSystem (included in 1.4) for an amazing file library. I often find myself reimplementing parts of it in my Ruby apps.
No matter which Smalltalk you choose, you must check out http://www.world.st/. It's a clearinghouse for every book, video, forum for all the variants of Smalltalk