Well the question has largely died out at this point if only because there are now a significant number of people who define an encyclopedia as “something like wikipedia”.
Personally what I found more interesting was the environment around the question. The first was that as far as I could tell prior to wikipedia no one had spent much time trying to define what an encyclopedia was. This was despite the massive range of items being lumped under the term. From the Yongle Encyclopedia through Encarta through the Dorling Kindersley Children’s Illustrated Encyclopedia to the Guinness Encyclopedia Of Popular Music. That’s quite a range of items. Of course this was largely because no one was very interested in the question. If you try and argue that the Dorling Kindersley science encyclopedia isn’t an encyclopedia no one is going to notice you are care if you do.
So why was the question raised when wikipedia turned up? In part it was because wikipedia an an encyclopedia that was widely read something that was previously rather uncommon. It was also an attempt at a defence mechanism by, oh shall we call them traditionalists. I’ll come back to the former but lets look at the latter. The obvious thing is that it failed utterly. Ultimately the general public doesn’t care about the rather academic debate which appears to even at its widest extent only to have impacted a small section of academia and hangers on and an even smaller percentage of the web community.
The perhaps more interesting trend was that “is wikipedia an encyclopedia?” tended to be a proxy for “is wikipedia Britannica?”. Even more than that it tended to be a proxy for “is wikipedia Britannica as the public generally perceives it?”. This results in a highly misleading debate since Britannica as generally perceived is a PR construct. Which is fair enough. Britannica is ultimately a for profit publication which has to advertise even if some of the impressions people get are somewhat misleading (see The Myth of the Britannica by Harvey Einbinder among other works). One obvious issue is the impression that Britannica is written by experts. The true siltation is more complicated. While Britannica has always employed headline experts for a long time the greater bulk of its articles were written by generalists in the form of librarians and journalists. Later on they pulled the same trick as journal publishers and used their prestige to get post docs to write their articles cheaply in return for the CV entry. A fairly reasonable approach since getting an expert on the English civil war to write an encyclopedia article (rather than a full blown biography) on George Goring article would probably be overkill. This does though mean that wikipedia is being compared to something that doesn’t actually exist.
To bring this meandering to a close and answer the initial question I argue that wikipedia is an encyclopedia simply because any definition that excludes it but includes all the other things commonly considered encyclopedias has in all probability been designed with the intention of excluding wikipedia.