Just reading through this thread and saw the bit where you said you wanted to add a simple maths question to weed out the bots.
This won't work ... maths questions are actually very easy for bots to solve. It won't even slow them down.
Far better, and could even be better than using reCAPTCHA is to use a simple "Question and Answer" system where the answer cannot be Googled.
I administer a website and forum running phpBB3 (just upgraded to 3.1.8) and I use the Q&A CAPTCHA system in there.
Here's an example question:
Please re-enter the following text, leaving out all the spaces and CAPITAL letters:
gEEks To Go is a GrEAt WEBsite
For me, I've had one human spammer over the past 2 or 3 years. It stops bots dead.
Also, it's accessible - ReCAPTCHA (and any visual CAPTCHA system) may also prevent visually-impaired folk. My eyesight is fine, but on one occasion I decided to listen to a ReCAPTCHA - it may have been changed now, but they put in so much noise that it's just horrible. Unusable, in fact, and my hearing is fine.
If you want to see it, take a look at the registration form at the forum: http://www.goosedale...yingclub.co.uk/
I know you're not running phpBB, but take a look here for some information - the ideas are portable enough:
Also, XHTML seems to have gone nowhere. See if you can focus on HTML5 instead, that's the modern way. It focuses more on functionality (what browsers can and cannot do) rather than strict standards. I played with XHTML once before, and I found it to be a right nightmare.
Hope this helps anyway!