A new vulnerability has surfaced with a proof-of-concept exploit.  So far, there are no documented reports of this being exploited in-the-wild. Users should remain cautious with an untrusted email attachment, just in case this is spammed by email later.  Microsoft is working on patches for Excel as noted in their blog entries.

Microsoft Information
http://blogs.technet.com/msrc/archive/2006/06/20/437826.aspx

Microsoft Office Long Link Buffer Overflow Vulnerability
http://secunia.com/advisories/20748/
http://www.frsirt.com/english/advisories/2006/2431

QUOTE: 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.  The vulnerability has been confirmed in Microsoft Excel 2003 SP2 (fully updated). Other versions and Office products may also be affected.