IIRC from my time with the PlayBook, everything has to be signed, but there's minimal hoops to getting your own signing key (no fee). Unclear if you could load arbitrary apps that you haven't personally signed, never tried.

Old BlackBerry, everything had to be signed by RIM's servers, even for dev builds. Which is incredibly entertaining when their servers go down while you're trying to get a release out (those days are far, far behind me now)