Simon, the next major (X.0) releases will all be XHTML compliant. Likely XHTML transitional as there is still a few things that are still a little hard to do in the current browsers with XHTML strict.
As far as your project though, because all the output is generated using templates you could likely just make your templates XHTML compliant and be good to go with the current system.
The one exception is when the program adds <br> tags when the user hits enter in the "summary" or "content" fields, if these aren't changed already, we could do a minor release to have them output as XHTML compliant <br /> tags.
If you do decide to put a XHTML template set together, make it as compliant as you can and then run the published pages through a validator and let us know about any issues and we'll work with you to address them for a minor (1.X) release.
Note: For those who are curious, XHTML is just the name for the latest version of HTML. It's essentially exactly the same as HTML but requires that you have good form in your code. For example, you need to close all your tags, etc. There's also a few little interesting things such as when you have a single standalone tag that doesn't have a closing tag (br, img, hr, etc) you need to put a " /" at the end to let the browser (or XHTML parser) know that it's not an enclosing tag. For the most part, XHTML looks and works just like HTML. So... Don't be scared! We're not switching to some new markup language that nobody knows about. It's just HTML with all the t's crossed and i's dotted. : )
Dave Edis - Senior Developer