Currently I can not find any way to determine when a system was registered.  This information is present on the system's web page in RHN.

I have checked in both system.getDetails() and system.listSystemEvents() and so far no joy.

I suggest modifying system.getDetails() to provide this data:

Method: getDetails
Description:
Get system details.

Parameters:

    string sessionKey
    int serverId

Returns:

    struct (details)
        string "profile_name" - System's profile name
        string "description" - System description
        string "base_entitlement" - System's base entitlement label (enterprise_entitled or sw_mgr_entitled)
        boolean "auto_errata_update" - true if system has auto errata updates enabled
        boolean "location_aware_download" - true if system is allowed to get downloads based on location
        string "address1" - System's address line 1
        string "address2" - System's address line 2
        string "city"
        string "state"
        string "country"
        string "building"
        string "room"
        string "rack"

** NEW **
        string "registered_date" - Date that the system was first registered with RHN