Hi Eric,
Sorry for the late reply here. I don't check the other boards here often. But thanks for the welcome
Gaia is very cool. We use it a lot at our agency and developed a lot of major websites with that framework (Pepsi and Toyota spring to mind). I saw a keynote about it. It's pretty sophisticated. I like the idea, however never worked with it personally (as I don't do much flash). But they love it at my agency.
I really stress the importance of knowing PHP. The reason is pretty simple; with a very basic understanding, you can create almost anything you want. My own knowledge of php is very little, yet as long as there are databases involved I can make whatever I want. Mailinglists, simple cms'es etc. are really good things for PHP.
Besides that, if you are looking for a simple CMS to start with and to get your hands around PHP I highly recommend Wordpress. Its basically a blogging tool but with the introduction of templates, which can be assigned to different pages, and extensive category options, it's a very good CMS. I prefer it much more over drupal (which is a bit more advanced) or joomla (who is a real b*tch I really like to forget about…).
I'm currently porting the we can't dance website to Wordpress. Most of it already works within a few hours of work (I could actually use my old CSS files etc.) Wonderful system and highly recommended!
Flash btw, will be supported on the iPhone I believe. Adobe announced something that makes flash work on the iPhone with CS5. So it might be a special version or something else. I dunno.
On the HTML side, you always need separate stylesheets for mobile versions. Even better are mobile subdomains which are sparser in content. Depending a bit on the platform you want to support and what information you have to offer.
Cheers!