RT-N66U Kernel version is 2.6.23, EXT4 not official supported yet, it may work... ("EXT4 was released as a functionally complete and stable filesystem in Linux 2.6.28") according to:
http://www.dd-wrt.com/phpBB2/viewtop...r=asc&start=15
http://svn.dd-wrt.com/changeset/17355
The "256MB" from RT-N66U are really weird but soon we will see, i guess that's not the final product yet with 1GB annouced.