Less amusing, but most helpful and informative (and non-pseudonymous!), are agents Kristin Nelson at Pub Rants and Jennifer Jackson at Et in arcaedia, ego.
Per Jackson, I finally did my homework, with help from Wikipedia:
"Et in Arcadia ego" is a Latin phrase that most famously appears as the title of two paintings by Nicolas Poussin 1594–1665) They are pastoral paintings depicting idealized shepherds from classical antiquity, clustering around an austere tomb. The more famous second version of the subject, measuring 122 by 85 cm, is in the Louvre, Paris, and also goes under the name "Les bergers d'Arcadie" ("The Arcadian Shepherds"). It has been highly influential in the history of art, and more recently has been associated with the pseudohistory of the Priory of Sion popularised in the books Holy Blood, Holy Grail and The Da Vinci Code.
The phrase is a memento mori, which is usually interpreted to mean "I am also in Arcadia" or "I am even in Arcadia", as if spoken by personified Death. However, Poussin's biographer Andre Felibien interpreted it to mean that "the person buried in this tomb has lived in Arcad..(ia)"; in other words, that they too once enjoyed the pleasures of life on earth. The former interpretation is generally considered to be more likely. Either way, the sentiment was meant to set up an ironic contrast by casting the shadow of death over the usual idle merriment that the nymphs and swains of ancient Arcadia were thought to embody....The phrase is used as the title of the first act in Evelyn Waugh's novel Brideshead Revisited. It is also the title of the second major story arc of Grant Morrison's esoteric comic book series The Invisibles, which incorporates Poussin's painting, and appears as an inscription on a gun in Cormac McCarthy's novel Blood Meridian.
Hmm...I must have snoozed through that lecture during Art History. I remember the shepherds but not much else. (Hey, it was an 8 a.m. class. Or maybe it was right after lunch. Anyway, it was dark in there.)