Can you say fonts?
Windows TrueType fonts, all OpenType fonts, and to a more limited degree, Windows Type 1 fonts are cross-platform compatible (although Windows Type 1 fonts are not generally supported by MacOS, the CS4 applications will recognize and use them properly if they are installed in the Creative Suite's private font folder on the Mac). Macintosh TrueType, Macintosh Type 1, and Macintosh dfonts cannot be accessed under Windows or by the Creative Suite applications running on Windows. If you start off on the Mac, if you use the fonts that are incompatible with Windows, you will need to license the matching Windows versions of same (or preferably OpenType) to make the jump between platforms.
For cross-platfom compatibility, you should also be religious about use of the file suffixes in MacOS (such as .INDD for InDesign documents, .AI for Illustrator files, etc.).