FlightAware and other data providers all seem to be missing the NY to Denver leg of flight US6215 / UA1429 which departs and arrives tomorrow (June 16 2012). The flight is not cancelled and does show up on the US Airways website:
A user sent me their ticket stub, which showed the flight US6215 for that segment, but it appears the airlines may have reassigned the flight number due to bad weather and flights being moved around.
One thing you might not know is that flight numbers used for passengers are not always the same as used for ATC purposes. Example: A passenger may be booked on UAL1234 but the flight plan shows UAL432P.
Hopefully someone at FlightAware can help. I don’t know if they can because the different flight numbers used for ATC and PAX are not always intuitive.
@mduell Thanks for the detailed response. I didn’t realize that the codeshares change so frequently, or that the API can look them up for me provided I specify the date I’m interested in!