This is how i did it, your mileage may vary, it's pretty complicated, you'll need a lot of stuff:
1) get yourself a web server (i'm suggesting Apache, look for WAMP for an easy implementation to run on Winlose) AND a DNS server. Also, your computer needs a static ip. I'm not here to teach you how to all this stuff
2) grab a copy of the right file the PS3 checks http://fus01.ps3.update.playstation....updatelist.txt
for us and so on
3) edit it. Dead easy, just replace "3.2100" with "3.1500"
4) create the right path in your web server shared folder. For the us version of this thing you need to create "update" that contains "ps3" that contains "list" that contains "us" and then place the edited ps3-updatelist.txt in the directory
5) On to the DNS side..... create a master zone called "fus01.ps3.update.playstation.net" for the us. Then add to that zone a machine record with the same name, but your computer's IP.
6) easy part: set up the PS3 so that it will use your new DNS server, namely just put the computer's IP as both primary and secondary DNS server.
7) if you didn't do so yet, start the web server and the DNS server, do a connection test with the PS3. IF everything went well, you should just connect and have no updates to do.