After you install the Aperture 1.5 Trial version, you can't update to Aperture 1.5.1.
This is expected—the Aperture 1.5 Trial version cannot be updated while running with a trial serial number. If you buy Aperture from the Trial, it is possible to update to 1.5.1 after you have entered the full serial number.
There is one scenario where Software Update may offer the Aperture 1.5.1 update after you install the Aperture 1.5 Trial. If you install the trial version and run Software Update before launching the Aperture Trial for the first time, Software Update will offer Aperture 1.5.1. Do not install the update—it will render the trial serial number inoperative. If you have already updated the trial version, just delete the Aperture application from /Applications and then reinstall the Aperture 1.5 Trial.