I read a post from Dean Takahashi at VentureBeat that a hacker called pod2g
has hacked iOS4.1.  However, I would not upgrade to 4.1 until something official comes from Dev-team and on last check there is nothing yet but a warning NOT to update to 4.1.

And really readers I have been saying this for how long now?  NEVER update until its been out there for awhile and you are SURE you can benefit from it.  This update is suppose to make running 4.x on the 3G faster.  Based on the independent test I have seen so far, its better then previous 4.0.x versions, but 3.1.3 is still faster.  I am still running 3.1.3 because there is nothing in 4.0 that I cannot already do, and in the case of multitasking, do better with 3.1.3.

So, yes there is a jailbreak POC out there, but hold your horses for now.