guide obtained from PS3Hax.net
Bypass the PS3 Firmware Check

Remember the Wipeout Pure ?hidden browser? hack for the PSP? Maybe you do, maybe you don?t; in any case, this method to circumvent the PS3 firmware check works in a similar manner by hijacking DNS requests and redirecting you elsewhere. The concept is actually quite simple? Once setup (follow the instructions below) PS3 traffic will be directed to our server which hosts modified files to report back firmware 0.00.

These are the steps I took (on my Intel Mac using DarwinPorts):
Quote
1. Open a terminal window
2. Type ?cd /opt/local/var/db/dports/sources/
rsync.rsync.darwinports.org_dpupdate_dports/net/ettercap-ng?
3. Open ?Portfile? and add ??enable-plugins? to configure.args. Save & exit.
4. Type ?sudo port -v install ettercap-ng?
5. If everything installed fine ? proceed?
6. Grab this file and replace ?/opt/local/share/ettercap/etter.dns? with it
7. Boot your PS3, ensure it?s connected to the network
8. Start ettercap by typing ?ettercap -T -Q -M arp -i en0 // // -P dns_spoof?
(replace ?en0′ with whatever network interface you?re using)
9. Wait a few seconds and try going to the PlayStation Store ? that?s that.
The same can be achieved on a Linux or Windows box using the same tool, ettercap-NG ? grab the appropriate version here. With Windows you?ll also need WinPcap.

So, with that said and done, you can now:

* play PS3 games online without updating
* browse the PlayStation Store without updating

Very cool stuff!