How much does it matter what server side language is used for building a web app to take advantage of HTML 5? It seems to me that the ruby community will probably have the fastest uptake, and as a result the most support. Does that seem right? If I want to make a serious investment in HTML5, what server side language should I use?
It doesn't matter which server-side language you use, they all support HTML 5.
You just have to make sure you write good markup that can be validated.
You will gain experience writing HTML as you build more complicated web applications with server side languages. I'd focus on one of those first and let the HTML come to you as you need it.
One of the best ways to learn is by validating existing HTML and fixing all of the errors and warnings. This will help teach you the common mistakes made when writing HTML and get you more familiar with the rules.
As far as server side languages go, you have several options and there are tons of resources already online to help you make the right decision.
http://articles.sitepoint.com/article/server-side-language-right
http://forums.whirlpool.net.au/forum-replies-archive.cfm/90884.html
It's all about picking the one that is right for you. And, the best part is, if you pick a language that doesn't suit you, you can learn a new one later.
Also, here is some information on Books for Server Side Languages which may prove helpful.
It doesn't matter at all.
Whatever server side platform you use should not be based on what revision of HTML you will render. The two are completely decoupled. That is unless you opt for some framework that generates the HTML for you.
The server-side language you use has no bearing on HTML5.
Presentation frameworks, if you opt to utilize one, may take a while to take full advantage of HTML5.
I agree that the server side language may not matter much, though any framework you choose such as django or Ruby on Rails will hopefully have html helpers that you can use to build a view.
FWIW RoR 3.0 outputs valid HTML 5