The JailbreakMe exploit was, well, nasty. It made use of an apparent flaw in how iOS handles PDF documents (this bug shouldn’t affect other PDF readers, just the iOS one, according to Sophos).

Security expert Charlie miller characterized the JailbreakMe exploit as “very beautiful work,” and said that it’s ” scary how it totally defeats apple’s [sic] security architecture.” So while jailbreakers might be disappointed to see this hack go away, for everyone else it’s probably for the best. I mean, if a security bug can jailbreak your phone, there’s no telling what else it could do.

There’s no word yet on whether or not iOS 4.0.2 un-jailbreaks your phone if you’ve already done the deed, so if you’ve jailbroken your iPhone 4 then upgraded to iOS 4.0.2 we’d love to hear from you.
For more details, see the iOS 4.0.2 and iOS 3.2.2 pages on Apple’s support site.