Now that the iPhone 4 is official, it's only natural that a bunch of accessories for the new smartphone should start rolling out immediately.
The most common new accessories we're seeing now (and will definitely continue to see) are cases. The iPhone 4 has a dramatically different shape from the 3G and 3GS, with altered measurements and flat backside. Thus, you really won't be able to reuse any of your previous cases for this new phone.
Apple has developed their own case of sorts, which they are dubbing as a "bumper." Available in six shades, it's really only a bumper as it is a rubber casing that slips around the edge of the rectangular gadget. There are slots for the new metal buttons on the side. Yet, with glass panels on the front and rear of the iPhone 4, a side casing doesn't really seem like enough to protect the device entirely from tumbles like a grip case would. The $29 price tag also seems like a bit much.
Then there is Case-Mate, which usually has a large collection of cases designed especially for the iPhone, and they are already well ahead of the pack with a huge listing of iPhone 4 designs. Prices range from $14.99 to $79.99 (that high-end one is a battery extender case), but none have a definite release date yet. At least shipping will be free. Most of the designs are similar to previous casings, although that credit card/ID case model sure looks useful for going out.
A little piece of advice: Buy cases for the iPhone 4 in person, or at least see and touch them in stores before ordering them online. Yes, you'll have to wait until at least June 24 (just like for the smartphone itself), but it's really helpful to see how durable and sturdy the case is, as sometimes materials don't look the same (or are not described as clearly) as online product pages might depict.
Some of the other third-party accessories we can expect to see more of soon are additional screen protectors (those glass panels are prone to fingerprints) and headphones. There are a number of sets already out there ready to control the 3G and 3GS, and in all likelihood, those controls will still work with the iPhone 4, but we might not know for sure until the device is issued to the public. And if they don't, we can be sure that headphone set developers will start designing new models tailored to the iPhone 4.