You can jailbreak by downloading a custom firmware patch, I'll post one in the downloads section tomorrow, also you need to have the correct patches in order to create a custom lobby, which can only be achieved on MW2/Black Ops at the moment. As for online play, you can not connect to online services when on a jailbroken PS3 as Sony will detect the modified firmware straight away and ban your account. You may be able to file transfer over to a secondary PS3 and try to run online lobbies from there but I'll need to read up on it in order to help you more.
Hope this helps mate,
Sincerely, REVIVE