Had the same problem with simple dns. Ended up un-installing simple dns and trying the ethercap method but it also had problems. Fixed it by getting rid of both programs and using ps3 proxy download it
here
I installed it then went into my ps3 setup and changed dns back to auto and told it to use proxy. gave it the ip of my pc and tried again.
all u have to do is tell ps3 proxyserver the ip of ur pc. tick ps3 mode and press start then click the logs tag.
restart ur ps3 and when it does its inital check it looks for
http://fus01.ps3.update.playstation....updatelist.txt this should appear in ur log. copy this link into internet explorer and download the txt file to ur pc. edit it so that the line 1.5400 reads 1.5100 and save it somewhere.
then click the last tab in proxyserver and press add. a new line will appear, on the left side paste the same link and then on the right side click the dot and tell it where ur edited version is.
that should be it done. one thing to note is that if i make any changes on my pc then the ps3 doesn't see the change unless its reset. it seems to keep a record if the update check fails.
hopefully that will get u going. it got me on running 1.51 and i was able to send gripshift to the ps3 by replacing the demo link found in the logs for the downloaded .pkz that i had on my pc