I use the OPS0123O (AOF Initialization Complete) message to key off an IPL. Within the rule I use OPSINFO('PRODUCTSTARTS') to tell how many times CA-OPS has been started since IPL. If it is one then we assume it is an IPL. If it is more than 1 we assume it is a restart and then go from there. I have a REXX which collects the IPL information at this time and sends me an e-mail each time one of our systems is IPL'ed. It also records this in a reference file for use at Disaster Recovery if needed. One caveat that I did find in using this is depending on what you use to send the e-mail (I use JES2MAIL) and how much of your system is up when AOF Initialization is complete, it may be tricky to send an e-mail immediately. Since the information I want doesn't change until the next IPL anyway, I have a dynamic TOD rule to wait for 10 minutes (TOD *+10 MIN) before sending the e-mail.
I decided on OPS0123O because it was the first point that you could safely execute a rule