Ah - my mistake - I need to add the filter fallback_rewrite_css_urls as my CSS 
contains some elements which are CSS3.

That brings me to a question I guess, if combine_css is enabled, but one of 3 
CSS files cannot be parsed - does (or should) modpagespeed rewrite and then 
combine the 2 CSS files which can be parsed, and leave the third?

I have identified the issue here:

With

    ModPagespeedRewriteLevel CoreFilters

Set, also having the line:
    ModPagespeedEnableFilters fallback_rewrite_css_urls

in the configuration file (from an earlier version of modpagespeed), actually 
manages to disable that filter as it is now present in the CoreFilter set(!)

Really? If so that is completely broken. IIRC we have made some tweaks in that 
area so this is possible but I'll investigate. Thanks for the report, sorry for 
the issue! :)

Hmm - I have been looking more closely at my pagespeed.conf - for some reason I 
also had a     

ModPagespeedDisableFilters rewrite_images

directive in there - having removed this, the fallback_rewrite_css_urls filter 
works whether or not I include it expressly (this is on the release version of 
modpagespeed).

Oddly I never even thought to look for this, as the images were all being 
re-written anyway!