A few weeks ago I stumbled upon a blog post by Nina Paley, the cartoonist perhaps most famous for Sita Sings the Blues, that emphatically made the point that, with Creative Commons licenses, yes means yes. I don't think I could put it better myself.
Seeing as I'm not famous, I don't interpret these permission requests as a cry for attention, and I recognize that people do it with the best intentions, but I hate seeing people waiting on replies from me before using photos I've already given them permission to use. The fact is, dealing with permission requests is extremely low on my priority list, when I've got myriad projects for my job, and outside my job, also waiting on replies for me. Unlike requests for permission I've already granted, those projects are legitimately stalled until I reply. (I won't even get into the number of personal emails that sit unanswered for days, weeks or months, for lack of energy at the end of the day.)
I love getting emails that say "hi, I used your stuff in this way, come take a look/can I send you a copy?" It means the system has worked, the things I've made have found another life somewhere without being encumbered by legal barriers, and I can read/reply to those whenever I've got the bandwidth. Nothing is stalled until I reply.
I have these additional caveats (which make reuse even more free) on my Flickr profiles, but to reiterate here for the record:
Even though all of my licenses use "Share Alike" (which technically restricts you to using that very same license), you have permission to use any other Creative Commons license so long as it includes either "No Derivatives" or "Share Alike". For the sake of clarity, this means any of the following licenses:
I'm not comfortable with the possibility of a derivative of a derivative work being locked down by full copyright (please don't contact me to ask, the answer will almost certainly be "no"), but you have permission to use any of the licenses above for derivative works, instead of my Attribution-Share Alike license, if you prefer.
So please don't ask and wait for me to reply. I want you to use my creations for anything, that's why I used a Creative Commons Attribution-Share Alike license (and if I used a more restrictive non-commercial license, it's because of restrictions beyond my control, like the San Diego Zoo prohibiting commercial use of photographs taken on their premises). Happy remixing, with the permission I've already granted you.