I know United uses the 800 series for transpacific flights. Perhaps they operate a flight 800? Incidentally, a few years back when United was planning to operate their own JFK
service to compete with Continental, they initially planned to use 800/801 as the numbers for the new service. I don't believe they were used when the flight went into the schedules, however.
Regarding TWA800, there's a ton of conspiracy theories available on the internet, and some of the evidence presented to support the missile theory is pretty convincing. I haven't drawn my own conclusions yet, but there's some very interesting reading out there and I would suggest checking some of it out in your spare time.