How, technically, might the US have snooped on Lavabit?

Ars Technica interviews Ladar Levison, founder of the recently-shuttered secure-er email service. They focus on the logistics and architecture of fed snooping. Levison: "I don't know if I'm off my rocker, but 10 years ago, I think it would have been unheard of for the government to demand source code or to make a change to your source code or to demand your SSL key. What I've learned recently makes me think that's not as crazy an assumption as I thought."