mod_deflate twiking

Hello,

I'm looking for an optimal and cross browser compatible configuration
for various MIME types in mod_deflate.

To begin with, I already put in place a dedicated MIME rewriting rule
for XHTML to be sent as "test/html" or "application/xhtml+xml" depending
on the Accept header.

This rule does a nice job so fahr:

#Serve xhtml pages as text/html to those that can't do application/xhtml+xml

RewriteEngine On
RewriteBase /

# Http Accept header does not equal "application/xhtml+xml"
RewriteCond %{HTTP_ACCEPT} !application/xhtml\+xml

# Http Accept header does equal "text/html" OR "*/*"
RewriteCond %{HTTP_ACCEPT} (text/html|\*/\*)

# Requested filename is anything ending in .xhtml
RewriteCond %{REQUEST_FILENAME} .*\.xhtml

# Match any url. - is used for no substitution. Set mime-type to
text/html. L is for last. I.e. stop processing rules
RewriteRule ^.*$ - "[T=text/html,L]"


Now, regarding mod_deflate, I run this configuration:

<IfModule mod_deflate.c>
AddOutputFilterByType DEFLATE text/html text/plain text/xml
application/xhtml+xml application/x-javascript text/css
SetEnvIfNoCase Request_URI \.(?:gif|jpe?g|png|exe|zip|pdf)$ no-gzip
dont-vary
BrowserMatch "MSIE" gzip-only-text/html
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch "MSIE" brokenvary=1
BrowserMatch "Mozilla/4.[0-9]{2}" brokenvary=1
BrowserMatch "Opera" !brokenvary
SetEnvIf brokenvary 1 dont-vary
Header append Vary User-Agent env=!dont-vary
</IfModule>

This setup work well with Firefox, but documents sent to Internet
Explorer 6 and 7 do not get compressed. Is it related to the MIME
rewrite rule (above) for "application/xhtml+xml" MIME types ?
How to set up mod_deflate to let it compress all compressable documents
in a compatible way for various current browsers, _including for XHTML
documents_ ?

Regards,

--
Léa Gris