The issue of the legal effect of a concession by a presidential candidate actually does have potential legal effect in at least one area--the recognition of an "apparent" president and vice-president-elect under the Presidential Transition Act of 1963. This was an issue that arose during the Florida litigation/recount in 2000 with respect to the obligation of the General Services Administration to release resources related to a presidential transition.
I wrote up the history of the act, and the potential legal effect of a concenssion in a law review article soon thereafter, which is available here (later published in the BYU Law Review).
I argued in the article that the idea of a concession was not a legally-relevant concept, at least in the context of the act. And it is hard to see how or why a concession would have a legal meaning at all. In fact, if you recall, Al Gore originally called Bush and conceded the election late at night, but on his way to his concession speech he learned that he might still win and then called Bush back and retracted his concession.
More generally, many of the rights that arise in the context of elections are rights that flow to voters, not to the candidates. For instance, during the 2000 election the Democratic Party of Florida filed a series of challenges to the validity of certain absentee ballots submitted by servicemembers. For obvious political reasons, Gore did not want to be a party to that litigation. A concession by Gore should not effect the rights of the Florida Democratic Party to sue and if this resulted in Gore winning the state and a Gore slate of electors being named, it is hard to see why a Gore concession should have legally-binding effect. So to the extent that candidate's concessions could detrimentally impact voters' legal rights, there is no basis for believing that a concesison should have predominant legal effect.
So, in thinking about Eugene's post, I'd submit that would be the way to think of it--did Gore's original concession to Bush have any legal effect? If so, can it be retracted? If it can be retracted, how could it have any legal effect? Does Bush have to rely on it (like an estoppel theory)? Would a third party have to rely on it? If so, who? What if the concession was private and not public? What if public but not private? What if Gore was on his way to give a public concession speech but was killed in a car accident and was never able to concede?
What I think this string of questions raises some questions about both the logical and practical coherence of vesting concessions with legal effect.