Developers are working hard to get a solid jailbreak running for iOS 6 on different iPhone, iPod Touch and iPad models, and so far had a little bit of success--there is a jailbreak, but a lot of apps and Cydia don't work just yet.
Although one hacker in Russia managed to figure out a way for making the 3D mapping feature run on an iPhone 4--a feature only available on the iPhone 4S, and the second and third generation iPad. Anton Tiktov (also known as iTony) tweaked the firmware of a jailbroken iPhone 4 and installed his own, third party app called 3DEnabler in order to get 3D map viewing working.
Sadly, there isn't much explanation yet into how 3DEnabler, well, enables viewing Maps in 3D. One thing it can't do, which iPhone 3GS and 4 owners might want though, is Turn-by-Turn navigation. You also can't use Cydia to help install the app due to the software not working with iOS 6 (for now!).
However, the iGuides Forum details how Anton manages to get around this issue, which entails a more manual installation--you can do it yourself in two tricky steps, but be careful.
The 3D view in Maps on the jailbroken iPhone 4 does look quite smooth and appears to load well, but it will be interesting to see how Apple will react to a hack like this--most likely some kind of patch to block it.