Mac OS X 10.4 has a native support for smart cards, called Tokend.

You can build and register such tokend for your card, and securityd will call this tokend whenever your card should be used by applications such as Safari, Mail, logon, ...

For more info:

How integrate OpenSC and Tokend?

Ideas from Stef:

This would allow use the use all pkcs15 functions on all available cards without any card-specific code in the tokend.

Comments wellcome...