I'm using Dell and I used to be able to get the serial using Powercli on 5.0, but I just recently upgraded to 5.1 U1 and I have noticed the array is blank now

Hardware.SystemInfo.OtherIdentifyingInfo

Not sure why this info is not populating in there.  If we don't get a response I'll hit up  Alan Renouf on twitter as I think he has a pipeline into the powercli devs and maybe he can shed some light on this.