Vista ships with "Password" and "Smart Card" Credential Providers. Some vendors are interested in writing custom credential providers. Check out http://shellrevealed.com/ for latest and up-to-date information on how to write Credential Providers.
Credential Provider Samples are available here: http://www.microsoft.com/downloads/details.aspx?FamilyID=1287ec56-77b4-48c4-8b58-35b7295d6c2c&displaylang=en. For comments or questions about these samples, please contact firstname.lastname@example.org.
If you are writing a custom Smart Card credential provider and would like to use the features such as the ones for removal policy, feel free to use the Group Policy settings for the corresponding registry key settings. Note that you will have to implement the corresponding logic in your code to appropriately log-off or lock work stations which may include removing cached information, etc.