Assuming the reported weather does not require an alternate airport, if an aircraft being operated under FAR Part 121 domestic IFR lands at its planned destination airport and does not have at least 45 minutes of fuel remaining, is this a violation of the regulations?
No, the regulation only concerns dispatching and taking off. Fuel burn will usually vary a little bit based on unpredicted wind shifts, performance variations and traffic deviations.
No person may dispatch or take off an airplane unless it has enough fuel -
(a) To fly to the airport to which it is dispatched;
(b) Thereafter, to fly to and land at the most distant alternate airport (where required) for the airport to which dispatched; and
(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption or, for certificate holders who are authorized to conduct day VFR operations in their operations specifications and who are operating nontransport category airplanes type certificated after December 31, 1964, to fly for 30 minutes at normal cruising fuel consumption for day VFR operations.