Come January next year, Mac IE will be no more.
Work on the Mac version of IE is believed to have been effectively halted back in February 2003, when Microsoft decided to can development of version 6.0 of the browser a month after Apple announced its own browser, Safari. Ironically, IE:Mac won plaudits from its earliest days for its speed relative to the rather clunky Mac implementation of Netscape. Indeed, the Mac OS 9 version of IE runs at a fair old speed, very different from the Mac OS X release.
[The Register]

This is the official line from Microsoft:

In June 2003, the Microsoft Macintosh Business Unit announced that Internet Explorer for Mac would undergo no further development, and support would cease in 2005. In accordance with published support lifecycle policies, Microsoft will end support for Internet Explorer for Mac on December 31st, 2005, and will provide no further security or performance updates.

Additionally, as of January 31st, 2006, Internet Explorer for the Mac will no longer be available for download from Mactopia. It is recommended that Macintosh users migrate to more recent web browsing technologies such as Apple's Safari.
For those of a masochistic bent, the latest variants of IE5 for Mac OSX and OS9 can be downloaded from links at top right of this page.