I have seen these as well. xMambo looks like a very good initiative. It would be nice to delegate more control over a sites rendering to CSS.
Not really sure why it is being touted as being more xhtml compliant, but Mambo is xhtml transitional so perhaps xMambo takes it to the next level. From what I have seen of the two specs (transitional vs. strict) it looks like strict would load faster due to have a more consistant tag base and rule set.
There are CMSs out there that are entirely based on the newer standards, although I can't think of one right off the top of my head. I have checked several out and decided to stick with Mambo due to the direction the team has plotted, the maturity of the code, availability of third party add ons (hundreds), and stability/security of the existing code base.
You should probably try to get a handle on the site visitors you plan to serve. This was the real deciding factor for me. The vast majority of my site visitors are still using some varient of explorer, and it is not real compliant with regard to properly rendering or parsing to the W3c specs. Foxfire sure does a lot better job.
Reliance on the newer specs (CSS2, xHTML strict) would sure lead to faster and better rendering of sites, but would also leave quite a few of my site visitors out in the cold who do not have browsers which can properly render them. I chose not to pursue either of these due to this limitation.
This is a great discussion topic, but one which I really think should be placed in the context of support for these specs within the greater community of available browsers.
Just my opinion btw...

, and we all know what that is worth.