[Swordmaker]

According to Microsoft, this flaw exists in the Mac OS X version of Excel as well as all current PC versions. However the flaw works by creating a buffer overflow.

The vulnerability is caused due to a boundary error in hlink.dll within the handling of Hyperlinks in e.g. Excel documents. This can be exploited to cause a stack-based buffer overflow by tricking a user into clicking a specially crafted Hyperlink in a malicious Excel document.
Secunia Excel Security Report

This is not as serious a flaw for Mac OS X users. Unlike Windows, OS X does not permit execution of any executable from the buffer... so the most it can do is to crash Excel on a Mac.

[Swordmaker]

...execution of any executable from the buffer...

Oops, make that "Execution of any executable from the stack."