Just read an interesting article by yahoo's Nicholas C. Zakas: What’s a web browser?
How do you best balance implementing new technologies like HTML5 and CSS3 while keeping support for browsers like i.e 6, is it a practical approach to day to day web development where deadlines must be met?
It depends on you audience, doing HTML5 and CSS3 heavily is acceptable only when you are sure your users will use only modern browsers. ( never been so lucky yet )
For now, you can consider them only for progressive enhancements. That is, you make a site working fine in IE6, and then you add features like round borders/shadows , the <video> tag or speeding up the site using local storage just as a bonus for the wiser users.
I'm starting a big project in few weeks, and all I can do is to make it HTML5 ready using the right doctype, the new tags and the few lines of javascript that will make it compatible with the old browsers, but will not use any cool feature for now.
I think it really depends on the type of site you are building as well as the type of intended end users. If you are building a site for a band, they tend to use a lot of flash/silverlight to make the interactions "cool" and more fluid. This would be an example of when to go all out with HTML5 and using canvas to accomplish some of the same effects. Obviously IE6 wouldn't work, but with more and more major companies dropping support for IE6 (Google included for gmail) it is becoming less and less of an issue as time passes.
If you need to develop a site where people will still heavily use IE6 then you might want to stay away from using the latest technologies. The pain you would go through to get things to work might not be worth it in the end especially if deadlines are tight and if you are not an expert.
Then there are the cases in the middle and here I think you need to think long and hard if you want to keep supporting IE6. I know a bunch of people who have dropped support and have found it much easier to strike the balance between CSS3 and HTML5 with the other browsers. It's still not perfect, but much easier to work with in the end.
On one hand, the market isn't ready for HTML5 and CSS3-only sites. However, on the other hand, if at least a bit possible, one should avoid support for IE6 and older (see the IE6-no-more movement for inspiration) because it's good for the industry and the average internet population to get rid of obsolete and problematic technologies.
If you wish to create a modern website I.E. HTML5 then I would recommend you take a look at http://html5boilerplate.com/ If you look at this boilerplate you will notice they take advantage of the modernizr JavaScript file to bring HTML5 support to older browsers.
I also strongly recommend you read Aaron Gustafson's Understanding Progressive Enhancement The idea behind progressive enhancements is that your site will look the best if viewed in a browser that supports modern CSS and HTML5 selectors/features while still providing a baseline for legacy browsers. There is a follow up article to Progressive Enhancements that is also worth a read http://www.alistapart.com/articles/progressiveenhancementwithcss
It depends on your traffic and who is spending money on your site. For us, it's necessary to continue supporting IE6 because a not-insignificant amount of money is spent by those users, and we're not in a position to compel them to upgrade. (If they're spending time upgrading, they're not spending time spending.)
If your site is not commercial then you should have a lot more freedom in determining minimum browser requirements.