A Word about Licensing
Licensing via the GPL isn't so much to protect our rights as developers but to protect the rights of people receiving our code. In a nutshell, it guarantees that anyone using our stuff has permission to do so, permission to change what we wrote, and permission to distribute those changes.
As far as I'm concerned, licensing code snippets here should follow the same model, if not a more permissive one. You're posting your code for all the world to see, and all the world should be able to benefit. If not, you should re-evaluate what you're posting and why.
Which License to Use
For most of my public code (i.e. the stuff I don't actually sell) I use the MIT license. It's actually less restrictive than the GPL as it's not viral, but still extends the same rights of use, modification, and redistribution. I've seen others claim their code snippets as published in the public domain in order to do the same - if it's owned by the public, the public can do whatever they want.
The thing to remember, though, is that content on this site is licensed under the Creative Commons Attribution-ShareAlike license (the cc-wiki logo in the bottom right corner). This license is not actually compatible with the GPL ... so unless you make other specific statements when you write code, technically people shouldn't be using your stuff in GPL'd plugins and themes anyway.
So if you want to be specific, make a comment whenever you post a code snippet explaining which license you're using with, perhaps, a link to a blog post or something explaining why. I personally recommend you use the most permissive license you're comfortable with. As I said, I use MIT for a lot of my stuff (so my name stays tied to my work), but some use public domain to cut out any illusion of restrictions.
Contributions From Others
Here's where the slippery slope begins. It's one thing to claim a license for your own work. But since others can modify your code on the site, they can (accidentally?) become contributors to your work. My argument would be that whatever license the original author applies to the code will be the binding one, and any later edits/contributions would fall under the same.
If an author isn't applying a license when they write code, then the CC BY-SA license for the entire site would likely apply. So if you're adding edits, revising code, changing formatting, etc, remember that your edits would be tied to the same.
This is where a generic "all my stuff is GPL" on your profile would do more harm than good. Yes, you might intend all of your code to be GPL, but you can't apply the GPL on code already licensed as CC BY-SA. So if you edit someone else's post, your code contributions there wouldn't be GPL and your catchall profile statement might lead someone down the road to think otherwise and unintentionally violate a license.
I would encourage including a single line of text with each GPL'd contribution just to keep things crystal clear:
This code snippet is licensed under the terms of the GNU GPL, version 3.