I spotted this in my Software Update. Here's the scoop directly from Apple...

Aperture 1.5.3 addresses issues related to overall reliability and performance in a number of areas, including:

• Generation of thumbnails for adjusted images
• Entering and exiting Full Screen mode
• Working with large sets of keywords in the Keywords HUD
• Restoring from a vault

Among the specific issues that have been addressed:

• Previews now update properly when images are sent to an external editor.
• Leaf Aptus 22 and Aptus 75 images are now imported with the correct orientation.
• When folders are imported as projects, the folder structure is now correctly preserved when identically named subfolders are included in the hierarchy.
• Reconnecting referenced images that have been externally edited now works more reliably.
• Setting the ColorSync profile in the Aperture Print dialog now correctly suppresses color management settings in the Mac OS X Print dialog.