AppleInsider has an article explaining why Microsoft will be shipping Office 2008 for the Mac without some of the applications found in the Windows version. The paragraph regarding Visio's absence is a bit odd:
In 2000, Microsoft paid $1.3 billion to acquire Visio Corporation, which had delivered a clone of the Lighthouse Design Diagram! application for NeXTSTEP. The Omni Group delivered a similar product for the Mac called OmniGraffle, which Apple bundles on new Macs. Omni also offers a pro version, which can open and save Visio 2007 documents, leaving little reason to want a port of Visio in Mac Office.
I am not sure what to make of the first sentence, never having heard the "clone" claim before. According to Wikipedia:
Lighthouse Design Ltd. was an American software company that operated from 1989 to 1996. Lighthouse developed software for NeXT computers running the NeXTSTEP operating system.... In 1996, Lighthouse was acquired by Sun Microsystems.
Two of the first products developed at Lighthouse were Diagram! and Exploder. Diagram! was a drawing tool, in which objects are connected together using "smart links" to construct diagrams such as flow charts. Diagram was copied by Visio Corporation, who were acquired by Microsoft in 2000.
A PDF of Diagram!'s spec sheet is available here, dated 1995, and it does read like a Visio features list. (Visio first shipped in late 1992.)
In any case, the AppleInsider sentence could use a rewrite, since it reads like Visio delivered its "cloned' diagramming software for NeXT. Visio was Windows-only.
The other item that surprised me was OmniGraffle's apparent ability to open and save Visio documents. Because of the way Visio is structured (through a spreadsheet-like database called the "shapesheet), it is not possible to translate Visio .vsd files directly. Checking the Web site indicates that a workaround is indeed employed: OmniGraffle uses Visio's XML import and export, which is like using DXF to get at the data in a CAD drawing.