If the iPhone 3Gs get iOS 5, you will be able to install it. 🙄
If not, then you won't get it on there by hacking. Plus, do you really want iOS 5? Remember what iOS 4 acted like on the 3G. 😉
If it is released for the 3Gs, I'll definitely be waiting until I can make a custom baseband preservation IPSW. And then if I don't like it, I'll be restoring to 4.2.1 with blobs. 😛