What firmware do you have?
If it is 1.5, you're in the clear.
If you have 2.0, there are downgraders, just look for them.
If you have 2.1 onward, you're completely screwed firmware-wise.
However, if you happen to own a copy GTA: Liberty City Stories,
there is an exploit that somebody took advantage of here:
http://www.hackaday.com/entry/1234000243073512/
That's all the help I can give you.
I'm stuck with 2.6 and no GTA so I have no options until a good downgrader comes along.