The C-SPAN web site's support for WMV on Mac (via Flip4Mac) is broken. Since many people may not want to install Real Player (the other option on C-SPAN), C-SPAN's broken web site means those Mac users can no longer stream live video from c-span.org.
But the C-SPAN streams themselves actually work just fine with Flip4Mac -- it's the broken web page at C-SPAN that is preventing Mac users from getting the streams. Until/unless the C-SPAN site is fixed, here are the URLs for the current C-SPAN streams (obtained from a Windows PC):
Mac OS X Hints