Today on the topic is forgotten root password to Linksys with Whiterussian on board.
Unfortunately howtos on www.openwrt.org doesn't work in 100%, so as usual in such case I need to find complete answer step by step using: google, "basic" linux knowledge and pinch of luck ;).

Well after more google study I find out a solution:

1. Press reset button after DMZ led turns on, hold approx 2s not less or more ;)
2. login using telnet on 192.168.1.1
3. unlock partition and mount it:

mtd unlock /dev/mtd/4
mount -t jffs2 /dev/mtdblock/4 /jffs


4. modify /jffs/etc/passwd file, removing root's password ie. ;)

That's all folks :)