When should I consider migrating to HTML 5?

Sarah,
It depends on which of the new features you want to make use of and if you are willing to write conditional code to direct users of old browsers to a separate page, or use javascript and CSS to generate code that will work cross-browser. It is definitely possible to create pages that work in older browsers if you do some work, so really, it depends on how much you want those new features, and how integral they are to your website functionality.

I'd recommend experimenting with some cross-browser pages now to try out the new features, but waiting until browsers offer more consistent support to roll out an entire website upgrade - unfortunately, this probably won't happen for a couple more years (or more!). And of course there are all those users who still use ancient browsers (like IE6!) that you'll always have to have an option for, even if it's not a great experience.

Some features (like <section>) are ignored by older browsers so incorporating some new tags now for primarily *semantic* reasons could be helpful, as long as your CSS is written so that browsers that ignore those tags still layout the page appropriately.

In the meantime, you can work on transitioning your pages to HTML 4.01 strict if you haven't already, which will ease your transition to HTML 5 in the future.

You can already use the most promising HTML5 element canvas in all modern browsers. For instance, the Javascript library Raphael adds Flash-like functionality without a plug-in, and is cross-browser.

As for the other HTML5 elements, take a look at this summary of HTML5 elements and this post on StackOverflow about HTML5 advances, then decide if you really need them yet.

It might be best to wait until the HTML5 standard is at least settled, since there is some wrangling at present, and support for the proposed HTML5 semantic tags is still poor and would be complex to manage until browser manufacturers have a clear spec to work to.