Oooooh, WHAT THE FUCK?
Used the config file from 2.6.17, did a "make oldconfig", answered to 20-sh questions (about new stuff), only to find out that it panics because it can't find root FS..!? Hmm, booted the old kernel, did menuconfig, only to find out that CONFIG_BLK_DEV_PIIX was moved to another option tree and wasn't properly selected as "Y" from the old config, so SATA wasn't available. When I fixed that, I found out that NAT and most of previous iptables functionality (from the previous kernel) simply.. wasn't there. After some Googling, I found out that the kernel devs made a major reorganization of the option trees, for some unknown reason (to me at least). The jump was almost like from 2.4 -> 2.6..