As reported earlier on MacNN, Apple has now posted the Aperture 1.5.1 update, now available for download here. To download this update you will need your serial number and must be running OS X 10.4.8 to install it.
Improvements in this new version include addressing numerous issues relating overall reliability and performance improvements. Keywords, Loupe, Cropping, Previews, Metadata presets, Versions, File renaming, Watermarks, Image deletion and iPhoto Library import, as well as some other features, have all received fixes. Of note, if you own a 1.8 GHz iMac with NVIDIA GeForce FX 5200 graphics card, Aperture now properly validates and runs OK.
For more detailed information about this update, visit this KB Article.