Run script in OS X 10.6 on network connection. (like /etc/network/if-up.d/)

A launchd agent watching /etc/resolv.conf, and two network related .plist files under /Library/Preferences/SystemConfiguration/ seems to work for me (in Mac OS X 10.8.4):

<?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>Label</key>
  <string>ifup.ddns</string>

  <key>LowPriorityIO</key>
  <true/>

  <key>ProgramArguments</key>
  <array>
    <string>/Users/Shared/bin/ddns-update.sh</string>
  </array>

  <key>WatchPaths</key>
  <array>
    <string>/etc/resolv.conf</string>
    <string>/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist</string>
    <string>/Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist</string>
  </array>

  <key>RunAtLoad</key>
  <true/>
</dict>
</plist>

I had previously only used /etc/resolv.conf, but there were cases where that wasn't enough.

You could try MarcoPolo or one of the similar utilities listed on its website. Location Changer looks promising if you're a minimalist.

You should consider using crankd, which precisely allows you to run scripts in response to many system events such as network changes, filesystem activity, application launching, etc.

As I couldn't find any sensible documentation, I also wrote a small blog post on getting started using crankd.