kaled - 11:57 am on May 27, 2010 (gmt 0)
There is no point serving HTML 4 code with an HTML 5 doctype. When you add features that require HTML 5, change the doctype.
However, you might consider test-validation with an HTML 5 doctype if you are concerned about not using features that will become deprecated (or even dropped). I haven't looked closely enough to determine if this is an issue or not.