bahamat's solutionbahamat's solution is correct; however, please note that the only way for me to make this work was to disable the rp_filter for every interface in the system, not only the two (eth1 and wlan0 in this case) involved in the NATing.
for f in /proc/sys/net/ipv4/conf/*/rp_filter; do echo 0 > $f; done echo 1 > /proc/sys/net/ipv4/route/flush (see the IMPORTANT note at the end of this page: Advanced Routing Howto -- the link posted there doesn't exist anymore, but I found it through the wayback machine)