I've been looking into this recently, it's possible for both base and pro models if they're under firmware version 9.0, anything above that is not yet exploitable, plus it's a softmod so it's a straightforward process, although it's not persistent across reboots so it's most convenient to leave it in sleep mode rather than power off. I think psx-place is one of the big forums for these things, but all the information is pretty readily available around the place. Good luck with it!

Edit: Turns out a kernel exploit for firmware up to and including 11.00 was released on April 30th, however no one has yet made a jailbreak for it. So while it will be jailbroken soon, currently only up to 9.00 is supported.