Hi,
It looks like that openwrt is based on wrt54gs gpl code version 2.07.1 released earlier this year. last month, linksys released the wrt54g version 2.04.4. Although the linux kernel is still 2.4.20, it adds quite a few things. Most noticablely, it has more protocol specific connection tracks in netfilter, e.g. SIP. It probably makes more sense to use this as the base.
Thanks,