To redirect requests for www.example.com to example.com (without the www) put this in your .htaccess:

RewriteCond %{HTTP_HOST} ^www\.example\.com$ [NC]
RewriteRule ^(.*)$ http://example.com/$1 [R=301,L]

And to do the reverse (redirect non-www to www), try this:

RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [R=301,L]

The following mod_rewrite rules will serve HTML files as application/xhtml+xml to supporting browsers.

RewriteEngine on
RewriteBase /
RewriteCond %{HTTP_ACCEPT} application/xhtml\+xml
RewriteCond %{HTTP_ACCEPT} !application/xhtml\+xml\s*;\s*q=0
RewriteCond %{REQUEST_URI} \.html$
RewriteCond %{THE_REQUEST} HTTP/1\.1
RewriteRule .* - [T=application/xhtml+xml]

Source: Mark Pilgrim, The Road to XHTML 2.0: MIME Types.