A joke and a question

[slovami.livejournal.com]

(x-posted from my journal) Today in my Russian class, our professor told us a joke that I found pretty funny. I thought I'd repost it here, because trying to figure it out can be a good grammar exercise for intermediate Russian learners.

It's a Vasiliy Ivanovich and Petka joke, a genre of jokes that I'm sure native speakers can explain. That's not really important, though, except to know that Vasiliy Ivanovich is V.I. Chapaev, a hero of the Civil War, and Petka is his sidekick. So the joke takes place in the context of the early years of the Soviet Union.

Василий Иванович заходит к Петьке, а тот сидит, что–то пишет. Василий Иванович говорит: "Петька, что ты делаешь?" Петька говорит: "Оперу пишу." Василий Иванович говорит: "О–о, а я и не знал. Много уже написал?" Петька говорит: "Много! Опер будет доволен."

Before you read the explanation, here is a hint: it has to do with ambiguous case endings.

( Translation and explanation behind the cut! )

And a question: is there a list somewhere of consonant clusters before which в and с become во and со? For example, в Москве, но во Владивостоке, во сне; с малиной, но со сметаной, со вкусом. I used to know them, and I've forgotten. Thanks in advance!