The real question here is, how soon is "soon?" Apple has been "looking into" fixing iOS 4 for iPhone 3G owners for nearly a month now. While the issues are not universal, many iPhone 3G owners have reported extreme sluggishness and crashiness that renders an iPhone 3G almost unusable after updating to iOS 4 -- and the problem's widespread enough that parody videos have been making the rounds on YouTube for months. While there have been numerous solutions offered from troubleshooters (disabling Spotlight seems to be a popular choice), these solutions haven't worked for everyone. Although there are methods to roll an iPhone 3G's firmware back to OS 3, the fact that Apple doesn't officially support these methods has kept many users from taking the plunge.
Speaking from personal experience, none of the 4.1 betas restored my iPhone 3G to a pre-iOS 4 level of performance. On the other hand, after giving my wife the iPhone 3G and setting it up as a new phone (rather than restoring from backup), she hasn't seen nearly the same level of problems I suffered through.
At the very least, the fact that Apple's CEO has acknowledged the problem at all should come as a good sign for iPhone 3G owners. Let's just hope that "soon" really does mean soon.
Apple iPhone 6s
Apple iPhone 3G