How to properly connect to a RN-171 Roving Network's WiFly module?
I was suspecting that maybe I was activating the SPI mode
  automatically, so I've putted the SPI associated pins at a state that
  will shut it down for this device (CS high, everything else low).


The problem was, indeed, the SPI module being activated, but not by the microcontroller's doings, but by some noise that was being generated in this. Sorry about asking a question with such a dumb mistake in hands. Thanks for all the comments.

To make this question (and answer) more useful, I can answer to @mng comment: it is possible to use SPI with the RN-171 module, but there is no documentation in their site yet. If you ask their engineers, they will send you some docs for you to use it, but you are not allowed to redistribute them.