There are modules in the netfilter distribution called ipchains.o and ipfwadm.o. Insert one of these in your kernel (NOTE: they are incompatible with ip_tables.o!). Then you can use ipchains or ipfwadm just like the good old days.
This will be supported for some time yet. I think a reasonable formula is 2 * [notice of replacement - initial stable release], beyond the date that a stable release of the replacement is available. This means that support will probably be dropped in Linux 2.6 or 2.8.