Thanks to the folks who wrote and recommended nameif as the solution for my kernel 2.6 interface swapping woes. It turns out that nameif isn't able to swap interfaces easily. In order to make it work you need to give your eth1 a bogus name, swap that interfaces MAC to eth0 and the swap the other MAC to eth1. Thanks (as usual) to Google and the debian-users list for the fix. Note that you cannot do this from an /etc/mactab, so you need to change /etc/init.d/networking (on Debian) and include the calls to nameif directly. You have to modify /etc/init.d/networking anyway because it doesn't check for an /etc/mactab and try to run nameif, and that's probably just as well.

At least I'm happily running 2.6 now.