I think you're looking for WordPress. To answer your concerns:

- it can do category-based feeds
- blogspam should be taken care of by Akismet
- templates are very customizable, so you should be able to serve whatever you want
- I'm not sure about the SVG
- it will import your MT data
- a very active community, which is awesome for finding support and plugins

Wordpress does category feeds by default. Plenty of spam solutions (Askimet works for me.)

Here's a plugin to make it output as application/xhtml+xml. Of course you can just customise an existing theme to your needs or write your own.

It'll import MT no worries.

Hosting it yourself is about 100x more awesome and powerful, but you could try looking at getting a blog on blog.mozilla.com (not sure what the policy is there) or going for wordpress.com. On blog.mozilla.com you can probably convince them to add themes/plugins for you. On wordpress.com I doubt it (ie. serving as application/xhtml+xml isn't going to happen.)

If you don't want to host it yourself, Wordpress has a hosting service as well:

http://wordpress.com/features/

You can use an askimet plugin with MT.
And category based feed are possible You need to creater a new archive template with the following code :

<?xml version="1.0" encoding="<$MTPublishCharset$>"?>

<rss version="2.0" 

  xmlns:dc="http://purl.org/dc/elements/1.1/"

  xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"

  xmlns:admin="http://webns.net/mvcb/"

  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">

<channel>

<title><$MTBlogName remove_html="1" encode_xml="1"$></title>

<link><$MTBlogURL$></link>

<description><$MTBlogDescription remove_html="1" encode_xml="1"$></description>

<dc:language>en-us</dc:language>

<dc:creator><MTEntries lastn="1"><$MTEntryAuthorEmail$></MTEntries></dc:creator>

<dc:date><MTEntries lastn="1"><$MTEntryDate format="%Y-%m-%dT%H:%M:%S"$><$MTBlogTimezone$></MTEntries></dc:date>

<admin:generatorAgent rdf:resource="http://www.movabletype.org/?v=<$MTVersion$>" />

<sy:updatePeriod>hourly</sy:updatePeriod>

<sy:updateFrequency>1</sy:updateFrequency>

<sy:updateBase>2000-01-01T12:00+00:00</sy:updateBase>

<MTEntries lastn="15">

<item>

<title><$MTEntryTitle remove_html="1" encode_xml="1"$></title>

<link><$MTEntryLink encode_xml="1"$></link>

<description><$MTEntryBody  encode_xml="1"$></description>

<guid isPermaLink="false"><$MTEntryID$>@<$MTBlogURL$<>/guid>

<dc:subject><$MTEntryCategory remove_html="1" encode_xml="1"$></dc:subject>

<dc:date><$MTEntryDate format="%Y-%m-%dT%H:%M:%S"$><$MTBlogTimezone$></dc:date>

</item>

</MTEntries>

</channel>

</rss>

You can even sign up for a free account on wordpress.com and I think it gives you all the features still.

If all else fails, I could host it for you (assuming that something.shawnwilsher.com would be ok for you)

XHTML is going away in the long run anyway. The current work of the HTML working group will supersede it.

(From Alex: I strongly disagree. XHTML 1 may live in future 1.x versions, but XHTML 2 is probably dead. XHTML is necessary if you want to integrate SVG, MathML, or other XML languages into the XHTML document - which realistically, I do want to do.)

I second wordpress.

They have a very good architecture, and you write templates in php+html, so you can do anything you want. It's extendable through plug-ins, which are actually really easy to write yourself if you wanna share the goods.
RSS feed templates are written in php+xml so you can do them however you want too.

I'd recommend wordpress.com if you don't want to host yourself. Won't be able to muck with HTML to much though, I guess you may be able to use SVG in posts (but not in the design).

Blog spam is a non-issue, it does categories, and I believe imports MT data.

If you want the control, you need to self host (wordpress.org). Still use akismet for anti-spam, and get all of the above plus 101% control.

I can recomment TextPattern. It's small and very easy to integrate into your own website

Converting WordPress to use application/xhtml+xml doesn't seem particularly safe, since it's not designed to generate XML properly – it can easily break when you accept user input.

If XHTML is a must, you might want to look at what Jacques Distler has done with his blog. Sanitizing user input for comments and stuff is particularly tough, but he seems to have done it.

I really have to recommend Drupal, not wordpress. I'm not sure if you are making progress with this but if you are having second thoughts about wordpress then let me know and I'll give you an account to play with Drupal, no commitments, if you like it then you get a free drupal blog, if not then no big loss ;)