Apple released a minor update to Aperture a few minutes ago, improving compatibility with the importing process of iPhoto libraries and a number of reliability improvements for brushes. The new version is labelled 3.1.2; Aperture 3.1.1 was released in December.
More information about the update are available here, full changelog below.
About Aperture 3.1.2
This update improves overall stability and performance, including specific fixes in the following areas:
Importing iPhoto libraries
Reliability and responsiveness when using brushes to apply adjustments
Reconnecting referenced master images
Aperture 3.1.2 is also available in the Mac App Store at $79.99.