The FCC has released a list [pdf link] of which US TV stations intend to shut off their analog transmissions in advance of the official June 12th date, together with a second list [pdf link] of which stations will slog it out until the very end. According to the current lists, 158 stations will transition before June 12th.
Different conditions have been set for stations depending on whether they are commercial or otherwise, together with their financial situation. The FCC requires stations to give viewers 30 days notice before ceasing their analog service. Non-commercial stations pleading low budgets are exempt, however, and may start shutting off transmissions as early as March 27th.
The FCC requires a minimum of an "enhanced analog nightlight signal" from at least one major network affiliate until June 12th, at least in major markets. The next expected shutdown date for commercial stations is April 16th.