I have an old ibook and gentoo insists on installing wpa-supplicant even if I have "-wifi" in my USE. There is no wifi on this computer, and as current ~ppc wpa-supplicant doesn't compile with gcc-4.3.1, this prevents the whole world update.
according to 'emerge -vta' wpa-supplicant is a forced dependance of networkmanager.
So you request wifi stuff to be optional, in the end...
I would suggest not using networkmanager on your machine then, because wpa_supplicant is integral to it working, even without wireless. Need to take it upstream if you'd like something like that changed.