Do We All Have to Learn HTML5 Now?
The short answer is no. First of all, new versions of the HTML specification do not make older versions obsolete. For example, the first home page I ever created looks the same in Firefox and Chrome today as it did in Mosaic and Arena in 1994. What's important is the assurance that the web pages we build today will look and function the same in another 15 years. We may update those pages for marketing and aesthetic reasons, but we will not be forced to edit them for technical reasons. Second, if you already know some HTML, it is not a matter of learning a new language or dialect, but simply incorporating new elements into your HTML vocabulary.
If you are a content creator/editor using Web-based tools to update web pages and post articles, you need to know that any HTML markup you use in a blog post, press release, or email newsletter will be the same in all your readers' browsers. It is best for you to stick with the elements and attributes of HTML4 until HTML5 has been more widely adopted and more guidance is forthcoming on how to use the new features.
If you design websites and keep up with tech trends on a regular basis, you will learn from your online resources about browser support for new HTML5 elements, which you can incorporate into your work with appropriate fallbacks and cross-browser testing. Now is the time to play with HTML5, while you reexamine your Web design and development methods. The HTML5 Web is collaborative.
If you manage a Web design company or development shop, your websites are probably sophisticated enough that you already do browser detection. My suggestion is to let one of your programmers become your HTML5 specialist, creating HTML5-aware versions of some of your in-development and existing websites.