Somewhere after R 2.1.0, I started noticing short phrases next to the R release number that is displayed on start up. This turns out to be a release nickname, an undocumented component added to the R.Version variable, accessible with R.Version$nickname.
The archives of R-announce show Peter Dalgaard announcing a new release's nickname on several occasions (e.g., here) but I don't find any other information. Despite being a list element of the object queried by R.Version(), it is not included in help file for that function.
Is there any documentation on this feature? Ideally I'd like a statement of the system used to determine a release's nickname, but any authoritative reference would be great.
In response to an email asking if there is a system to the names, Peter Dalgaard states there is
"No system (except that they should be in season at release time)"
Thus, they are not alphabetical or otherwise ordinal -- there is apparently not a way to infer the order of releases from their nicknames.
There is, however, the appearance of a possible general theme:
As no one has uncovered any documentation, I'll tentatively accept my own answer.