Thanks to William and all others who responded to this.
I have found what I was looking for (ironically while listening to U2's "I Still Haven't Found What I'm Looking For") at: http://msexchangetips.blogspot.com/2006_08_01_msexchangetips_archive.html
Scroll down (or if you're lazy like me Ctrl + F it!!) to "Exchange: Query Mailbox Creation Timestamp".
Some other recently useful stuff on this site, thanks to James Chong - a legend!!!
Yes, in Exchange 5.5 mailboxes were separate objects with an association to a user account. You could even have mailboxes without an associated user account. That is not possible in AD/Exchange2k*.
I thought I would add at this point that it was possible to view what time a 5.5 mailbox was created (in Exchange Administrator), just need to find out how to find the equivalent timestamp in Ex2k3.
From: exchangelist-bounce@xxxxxxxxxxxxx [mailto:exchangelist-bounce@xxxxxxxxxxxxx] On Behalf Of cs
Sent: Wednesday, October 25, 2006 3:21 PM
Subject: [ExchangeList] Re: Exchange 2003 Mailboxes
On 10/25/06, Michael B. Smith <michael@xxxxxxxxxxxx> wrote:
I can't find any supported way to do it, other than using direct MAPI calls.
I wouldn't mind being corrected either.
I don't know how to easily do that; I don't think it is a readily accessible value. Obviously, the mailbox is an attribute of the user in AD.
But I wouldn't mind being corrected.
I would expect somewhere, maybe with some logging in place at the time, we would have been able to record when a user became mailbox enabled.