Ascender Corp announced that Mac users who don’t have Office 2008 for Mac
now have a source to acquire a legal copy of them:
http://www.ascendercorp.com/pr/pr2008_04_17.html

Office for Mac 2008 includes the Microsoft ClearType Font Collection - 24
fonts including Calibri and Cambria. To ensure compatibility between Office
suites, later this year Microsoft will release an update to Office 2004 for
Mac with a free license for the fonts for use in Office 2004. But Mac users
who don’t have Office and want the fonts for use in Corel, QuarkXPress and
Adobe Creative Suite for design and publishing now have a way to license and
download them.

More info on the Microsoft ClearType Fonts can be found here:
http://www.ascendercorp.com/ctfonts.html