"An attacker which knows a connections client IP, server IP and server port can abuse the challenge ACK mechanism
to determine the accuracy of a normally 'blind' attack on the client or server."

Not going to be a common attack, then.

May affect other OSes.

https helps, but does not prevent a DDOS using it.

After reading the security alert, it seems that more recent versions of the Linux kernel do not properly keep track of incoming ACK segments, making it possible to fire a number of bogus packets to the server from an attacking computer in order to hijack an existing TCP connection between that server and a third party.

After that it is possible to inject malicious packets in the existing data stream between those two computers. Before this attack will work the attacking computer must know the IP addresses of the two parties who are actively communicating with each other at the moment of the attack. Therefore this attack is most useful to inject malicious packets in persistent communications between servers. Attacking a client connecting on an ad hoc basis with servers while surfing on the internet is much more difficult because the attack can only be successful during an existing TCP connection. DOS-ing the TOR network which uses a number of servers with fixed IP addresses is a feasible option as is mentioned in the page engine linked to.