Tahoma sort of used to work for me in the Archicad 12/Leopard combination with Office 2008 for Mac installed, but it would not render properly, substituting a different font. After upgrading to Snow Leopard, Tahoma renders as a series of rectangles in Archicad 12 and 13 if I open an old file containing the font. If I open an new document from our office template (which contains Tahoma) in either of these versions under Snow Leopard, Tahoma appears in brackets in the font list and again renders as a series of rectangles, but if I open a new document in 13 using Archicad's standard template, its not in the font list.
There is some discussion of this issue here.
http://archicad-talk.graphisoft.com/viewtopic.php?p=154144#154144
Roger