Base Safari version (without nightly build): Version 5.0.2 (6533.18.5)
Every Nightly since the base version (above) has exhibited this bug. Select any file link that would lead to a download, and Webkit crashes. I run Safari though the Webkit.app installed by the nightlies.
I am running 10.6.4 Build 10FS69 0n a 2010 (March) MacBook Pro 13" and 10.6.4 also on a 24" (August 2010) iMac.
This crash is 100% reproducible.
This should be fixed in the latest nightly.
*** This bug has been marked as a duplicate of bug 46380 ***