When an ultrasound shows no fetal heartbeat, the results are pretty conclusive as long as the pregnancy's dates are not in question.
A transvaginal ultrasound that is properly performed will not miss a fetal heartbeat if one is present -- the results are considered accurate and reliable. Thus, a transvaginal or abdominal ultrasound showing no fetal heartbeat will mean one of two things: either the pregnancy is too early along for the heartbeat to be visible (which is possible only if the measurements match a gestational age of 7 weeks or earlier), or a pregnancy loss has occurred. Note that this does not necessarily apply to handheld doppler devices, which do not detect the heartbeat until later.
Any time an ultrasound fails to find a fetal heartbeat after one has previously been seen, the doctor can conclusively diagnose miscarriage. In addition, when there is no heartbeat in a pregnancy that is definitely far enough along that the heartbeat should be visible, the ultrasound results definitely mean miscarriage.
Remember though that there are variations in when different types of ultrasounds are able to detect a heartbeat. A transvaginal ultrasound finds the heartbeat fairly early, usually between 6 and 7 weeks of gestation. An abdominal ultrasound will find the baby's heartbeat roughly one week later, or between 7 and 8 weeks of gestation. A handheld doppler ultrasound device (the ones OB/GYNs use during prenatal visits) may not be able to find a heartbeat until as late as 12 weeks.
Concerns Regarding Early Fetal Development. American Pregnancy Association. Accessed: Jan. 22, 2009. http://www.americanpregnancy.org/pregnancycomplications/earlyfetaldevelopment.htm.