Can I rename the wp-admin folder?

Unfortunately it’s not currently possible nor does there appear to be will to consider it as a modification as you can see by this recent thread on the wp-hackers list and this ticket on trac.

If you’d really like to see this be revisited I’d suggest:

  1. Present your case on wp-hackers but be forewarned your use-case better be good and not “security through obscurity” or it will get shot down as above.

  2. Present your argument in a trac ticket with the same caveats.

  3. Even better, upload a patch to trac that enables your desired functionality. It’s much harder to say no when the work has already been done (but of course, they do have a preference for saying “no” a lot more often than they say “yes” so be forewarned.)