I presume you mean preserving the Pantone colors as spot?
In Photoshop, you can save spot channels in a .tif file, but that doesn't appear to be an option in Illustrator. Doing an export as .tif gives me a color model pop-up menu with three options -- RGB, CMYK and Greyscale. (CS3 here)
Why do you need to create a tiff from an Illustrator file with spot colors? It seems like if you can't use a native AI file that .pdf or .eps would be your best options, since they're retain vector info instead of rendering a raster file.