Question says it all really? I can't really see what useful scope this site has if all of the system administration and web-design elements are stripped out.
This is covered in the respective FAQs for each site; see Rebecca's response here:
If your job title is "system administrator", your job related questions generally belong on Server Fault.
If your job title is "programmer", your job related questions generally belong on Stack Overflow.
If your job title is "webmaster", your job related questions generally belong here.
It's all about professional areas of expertise. Is there overlap? Sure, just like in real life, some programmers do sysadmin type things, and some programmers do webmaster type things. But that doesn't mean the distinction isn't clear.
In other words, just because snowboarders and skiiers both go downhill really fast on snow, does not mean they are the same communities.
From the short time here in PW, I've noticed that most of the questions being asked is more of technique than specific coding.
Pro Webmasters has a obscure scope, however it doesn't make it any less important. There are many web hosts, there are many third-party programs (MySQL, etc.), tons of CMS's, and many SEO techniques that are required for a dynamic website to compete in today's internet. I see PW is more of an advice site - for people who need questions to the subtle or behind-the-scenes features of websites.
Just as Jeff said, there are overlaps where questions could be placed in either StackOverflow or ProWebmasters - such as which to code first, HTML/CSS or apps? Being a Webmaster or even Web Developer, there is a vast amount of knowledge that could be considered the 'tricks of the trade.' ProWebmasters covers all of that.
My personal understanding of the different roles involved gives me a clue where each topic should go. Comparing the roles:
Webmasters vs. System Admins
Webmasters generally deal with web hosts and usually configure their own (or employers') websites via the web host's control panels, not server hardware or server software (e.g. Apache, IIS).
Sys admins work for web hosts and will deal directly with the server hardware and server software (e.g. Apache, IIS) configuring them and the control panel software for other multiple users / websites.
Webmasters vs. Programmers
Programmers / developers will be the guys who create the CMS and forum software that webmasters use. As web developers they will build big, complex sites using web frameworks like Zend or CakePHP (rather than just HTML/CSS).
Depending on which of the roles above you see yourself most closely fitting into above, you should post to the relevant site.
Just my take...