Very sweet idea, this package is like a dream come true. :-)
I checked out your code and looked at it very briefly, ran tests - great documentation, great examples.
There are small things I'd like you to change, for example, using protected instead of private so people can extend/overload your class and still access the properties and methods. Or making the code more robust by checking that $res is not "false" in File_CSV_Get::_parse(), etc..
If you keep as-is, think about file_exists() vs. is_readable(). I *think* on some systems there can be a difference, which can lead to a nasty error. ;-)
I'd also like to be able to "open" a URL which outputs CSV-data, and then I wouldn't force the filename/URL to end in ".csv". In general, some operating systems tend to omitt file extensions anyway, so in that case I'd assume the file/URL contains CSV-data and make File_CSV_Get::_validate() more robust and handle an error.
If you really want to stick to ".csv", I'd at least replace the check with "substr($file, -4)".
Aside from those the package.xml, tests and maybe the structure in your SVN needs some minor fixes, I can help you if you want. Those are just minors though. ;-)