I think first and foremost kind of metrics should be pertaining to Return of Investment from Automation.
Automation tools are an expensive proposition.
Actual Testing time saved through automation should be constantly monitored.
Rich I also like to use the following metrics as Linda says elsewhere in trend patterns for automation:
Time spent on maintaining test scripts.
Time taken to "smoke test" an environment/ build
Ratio of time spent on set up/execution/analysis
Time released from confirmatory testing to Investigative testing (ie Exploratory Testing)