While trying to find an appropriate way to invoke a custom script on system resume from suspend, we've found that previously existed /etc/acpi/resume.d directory is missing in Ubuntu 10.04.
The solution for this issue we found is to use pm-utils package functionality:
Add a script to /etc/pm/sleep.d/ directory. As a fist parameter the script should get the system event name provided by pm-utils. In our case it was either thaw or resume, depending on suspend type (suspend to disk or memory suspend).