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Maybe I haven't explained myself very well.
As far as I am aware the standard does not use the term ESSID and as such it should not be used ( even if it makes sense ).
SSID could identify a BSS ( let's not talk about IBSS in this post ) whether it is on its own or part of a ESS. If you set the same SSID in the APs which are part of the same ESS you are just giving a hand to the station client software to roam from one AP to the other when the station changes coverage area. But the APs in the same ESS could have different SSID which would not help the client software to select a new AP and roam but ( as the standard says) it still "appears as a single BSS to the logical link control layer at any station associated with one of those BSSs."
Check out this Power point presentation. If you have power point click here:

grouper.ieee.org/groups/802/21/doctree/2004-07_meeting_docs/21-04-0105-00-0000-what_ess.ppt

if not get google convert it for you here ( but you miss a lot of images )

http://66.249.93.104/search?q=cache:Ht7eYp-XeTcJ:www.ieee802.org/21/doctree/2004-07_meeting_docs/21-04-0105-00-0000-what_ess.ppt+Submission.+Anecdotal+Definitions+for+ESS&hl=en&gl=uk&ct=clnk&cd=1