Those wondering how to roll-your-own DTD might well be served by reading the W3C recommendation on the Modularization of XHTML. Helpful to know if you want to push the limits of XHTML.
On a related note, the W3C has also assembled a list of valid DTDs to use in HTML templates.
Next week, two colleagues — Joe Clark and Tara Cleveland — will be speakers at the Toronto Usability Summit. If you’re in town, and can spare the time, you may want to attend.