A few months ago (before the release of TL2013) I uploaded a script to CTAN.
What are the next steps (if any) that need to be taken to get the script or package part of the TeX Live and MiKTeX distributions?
For (La)TeX packages, there is really not much to do from the author's point of view: uploads are picked up by the TeX Live team and Christian Schenk, and are then added to TeX Live and MiKTeX, respectively (license-allowing). The TeX Live team provide an overview of their requirements: that I know of there are no MiKTeX guidelines, but following the TeX Live ones can't hurt in this case either. Assuming you've already uploaded to CTAN, key points are:
The TeX Live guidelines apply to scripts as well as packages, with the additional advice that Perl or Lua are the 'preferred' languages as they are bundled with TeX Live. MiKTeX doesn't have Perl built-in, so a Perl script in MiKTeX will require some action by the end user. That shouldn't prevent it going in to MiKTeX, though. There's also advice on naming: don't create any new conflicts!