Well, there are a couple of things you could do. You can talk the sysadmin of the box into putting a script in the startup routine to send an email, or you can do what you suggested and run a cron job and have it send an email. Either way, look into Mail::Sendmail.

As for system commands, a simple command like w or uptime will report uptime. You could get the outout with backticks and parse out the uptime. If it is less than a day, send you the email.

Good luck
Scott