I have some python knowledge but I'd never consider myself particularly fluent in the language. I've a potential machine vision project in the works that would benefit greatly from SimpleCV but I'd rather not have to be hugely fluent in python in order to actually use it from a time standpoint. It's not that I mind learning python; it's that I'd rather get the project done with and then figure out the rest of it.
Very little. Python has great online help
Defining functions is trivial, arrays are pretty obvious. About the only python-esque thing is probably using range[] to do a loop rather than for (i=0.....)
Just like in C++ all the new stuff about iterators and lambdas looks clever but isn't necessary to get stuff done.
If you are using this for matlab type, interactive image algorithm developement then ipython is a great environment. It also has a shell that can do inline image and plot display and shows the man page for each function as you type the name.