JailbreakMe PDF hole closed by iOS 3.2.2, 4.0.2
There needs to be some sort of vulnerability in order for a jailbreak to be created for a version of the iOS. In this case, however, the vulnerability found was a flaw in the way the Safari browser handled PDFs. It's a minor update, and quick to install, as we discovered when we tried it out.
Jailbreakers (and they should already know) should probably eschew the update until after a new jailbreak is created. Those users would (should) already have installed the protective workaround for jailbroken devices. There is also talk of a patch for jailbroken devices, as the work involved in jailbreaking a new OS version is probably more than that involved in a patch.