ikiwiki is not yet able to create a Google sitemap internally, so I'm using google-sitemapgen. To run it automatically when the website is being updated, I've changed the git hook to run it after the ikiwiki hook.

• In the ikiwiki setup file let git_wrapper point to a file which is not the post-update hook, so you're able to run it from your own skript. I'm using /path/to/myikiwiki.git/hooks/post-update.ikiwiki.
• Write a skript which runs as post-update hook and executes the created hook from ikiwiki and google-sitemapgen with a proper configuration. My versions of both files are listed below.

/path/to/myikiwiki.git/hooks/post-update

#!/bin/sh

/path/to/myikiwiki.git/hooks/post-update.ikiwiki

/usr/bin/google-sitemapgen --config=/path/to/mywikiconfig/sitemap_config.xml

exec git-update-server-info

/path/to/mywikiconfig/sitemap_config.xml

<?xml version="1.0" encoding="UTF-8"?>
<site
  base_url="http://bzed.de/"
  store_into="/path/to/bzed.de/sitemap.xml.gz"
  verbose="0"
  suppress_search_engine_notify="0"
  default_encoding="UTF-8"
  >

  <directory
    path="/path/to/bzed.de"
    url="http://bzed.de/"
    default_file="index.html"
  />

  <!-- Exclude URLs that end with a '~'   (IE: emacs backup files)      -->
  <filter  action="drop"  type="wildcard"  pattern="*~"           />

  <!-- Exclude URLs within UNIX-style hidden files or directories       -->
  <filter  action="drop"  type="regexp"    pattern="/\.*"     />

  <!-- Exclude ikiwiki directories -->
  <filter  action="drop"  type="regexp"  pattern="/helponformatting/*"           />
  <filter  action="drop"  type="regexp"  pattern="/ikiwiki/*"                    />
  <filter  action="drop"  type="regexp"  pattern="/markdown/*"                   />
  <filter  action="drop"  type="regexp"  pattern="/openid/*"                     />
  <filter  action="drop"  type="regexp"  pattern="/pagespec/*"                   />
  <filter  action="drop"  type="regexp"  pattern="/preprocessordirective/*"      />
  <filter  action="drop"  type="regexp"  pattern="/sandbox/*"                    />
  <filter  action="drop"  type="regexp"  pattern="/shortcuts/*"                  />
  <filter  action="drop"  type="regexp"  pattern="/smileys/*"                    />
  <filter  action="drop"  type="regexp"  pattern="/subpage/*"                    />
  <filter  action="drop"  type="regexp"  pattern="/templates/*"                  />
  <filter  action="drop"  type="regexp"  pattern="/theme/*"                      />
  <filter  action="drop"  type="regexp"  pattern="/wikiicons/*"                  />
  <filter  action="drop"  type="regexp"  pattern="/wikilink/*"                   />
  <filter  action="drop"  type="regexp"  pattern="/wmd/*"                        />


  <!-- Exclude css files, favicon and javascript -->
  <filter  action="drop"  type="wildcard"    pattern="*.css"                         />
  <filter  action="drop"  type="wildcard"    pattern="*favicon.ico"                  />
  <filter  action="drop"  type="wildcard"    pattern="*.js"                          />

  <!-- Exclude ikiwiki.cgi -->
  <filter  action="drop"  type="wildcard"    pattern="ikiwiki.cgi"                   />


</site>

See the examples and README files in /usr/share/doc/google-sitemapgen/ for an introduction into configuring google-sitemapgen.

So far the generated sitemaps works very well, especially for search engines which are not able to use the rss feeds like Google does.