I have worked back and forth with mixed results. As mentioned, ActiveX is not supported in Macs, VBA is about 50% crippled compared to the Windows version, VBA Reference Libraries are pitiful on the Mac side (yes, Word, PowerPoint, and Entourage libraries are supported for interchange). I have taken many of the VBA projects from Windows side and used directly without modification (or one line) for the Mac (PasteLink Charts into PowerPoint, etc.) and it has been fine.
You might want to check out three sites:
Jim Gordon is an MVP for Mac Office (puts him in world-class company).
So, also,
John McGimpsey is an MVP for Mac Office, and he has a large section on XL 2004 and some of the issues related to it.
And at
VBAX - VBA Express, check out the Mac section. BlueCactus is converting all of his Windows VBA code to Excel v. X for Mac (predecessor of XL 2004).