I just upgraded the calibre OSX build to use the newly released Qt 4.7. This should hopefully mean various speedups, particularly in the ebook viewer. Also The GUI is now Cocoa based rather than Carbon based (if you don't know what that means, it doesn't matter to you). Another new feature is that deleting books will now put them in the recycle bin.
In my testing the ebook-viewer and main user interface startup times have both fallen, with much larger improvements in the viewer.
For various technical reasons, upgrading to Qt 4.7 meant that I had to drop support for OS X Tiger and PowerPC Macs. The current stable calibre release will be the last version of calibre to work on Tiger and/or PowerPC. So if you are on either of these platforms, this beta is not for you.
As with any major upgrade, I'm sure some things broke. The current calibre release already switched to Qt 4.7 for windows and linux, with no major breakages, so hopefully, the same is true for OS X.
The Qt 4.7 based build is available here:
I am particularly looking for test reports from people running OS X Leopard 32-bit as the only OS X machine I posses runs Snow Leopard on 64-bits.