Upgrading from a 18.6 to a 26.2 kernel in Debian Sid broke madwifi due to high-latency packet response (up to 25000 ms) and high packet loss (40%.)  Downgrading to madwifi stable (0.9.4) or trunk (07/31/08) resulted in same behavior.

Reverting back to the 18.6 kernel and installing madwifi stable (0.9.4) resulted in proper function.