Design will be the same as iPhone4 physically, antenna will be retooled to have less attenuation ala Verizon iPhone4's antenna. -- seriously antennagate was an overblown issue that only affects a very small group of users (mostly US)
I doubt Apple will go with a single dual mode CDMA and GSM device. They will keep CDMA separate from GSM. It's unlikely that they will use LTE or WiMAX, adoption for those are still too low.
The 8Gb 3GS will be killed off. iPhone4 will be moved down loosing capacity and will also loose it's glass back and instead have a polycarbonate back.
I don't think they will change the size (bigger or smaller) footprint wise although they could change the screen size (resolution will be the same)
Camera would also stay the same
Processors & RAM will get a bump
White iPhone5 will be at launch
Anyway these are my initial speculation for iPhone5, what are yours?