Today I needed to look up the warranty info on one of my ESX servers -- I knew I had it in a spreadsheet somewhere, but it turned to be quicker to just look it up from the console using "dmidecode". The service tag is listed as the Serial Number under System Information.

This works for regular Linux servers too:

[root@myesxserver root]# /usr/sbin/dmidecode |grep -A4 "System Information"

System Information

        Manufacturer: Dell Inc.

        Product Name: PowerEdge R710

        Version: Not Specified

        Serial Number: Hxxxxx1