As designed, a MacOS X 10.5 Server running the SOftware Update service cannot provide OS X 10.6-specific updates to client machine, but there is a way to make it happen with a little hackery.
1) Stop SUS
2) Download http://swscan.apple.com/content/meta/mirror-config-1.plist and copy it to /usr/share/swupd/html/content/meta/mirror-config-1.plist
3) Change it to look like this:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>PrimaryCatalog</key>
<string>http://swscan.apple.com/content/catalogs/index.sucatalog</string>
<key>CatalogsList</key>
<array>
<string>http://swscan.apple.com/content/catalogs/index.sucatalog</string>
<string>http://swscan.apple.com/content/catalogs/others/index-leopard.merged-1.sucatalog</string>
<string>http://swscan.apple.com/content/catalogs/others/index-leopard-snowleopard.merged-1.sucatalog</string>
</array>
</dict>
</plist>
4) Edit /etc/swupd/swupd.plist and change the metaIndexURL entry to look like this:
<key>metaIndexURL</key>
<string>http://myserver.com:8088/content/meta/mirror-config-1.plist</string>
<key>portToUse</key>
5) Start SUS and wait for it to download the catalog files and to generate the local versions thereof.
6) Once it has generated the local catalog files add some symlinks like this:
cd /usr/share/swupd/html
ln -s /usr/share/swupd/html/content/catalogs/index.sucatalog .
ln -s /usr/share/swupd/html/content/catalogs/others/index-leopard.merged-1.sucatalog .
ln -s /usr/share/swupd/html/content/catalogs/others/index-leopard-snowleopard.merged-1.sucatalog .
7) Just in case restart SUS
8) Wait for SUS to download all the updates
9) On your 10.6 client go in the Finder to /Library/Preferences and open com.apple.SoftwareUpdate.plist. Change the CatalogURL entry to point to http://myserver.com:8088/index-leopard-snowleopard.merged-1.sucatalog
10) Launch Software Update on your 10.6 client and be happy