Some rumours of new hardware features for the alleged new model in the Summer:
32GB storage (an obvious upgrade)
Better CPU and/or GPU
3.2MP camera + autofocus [ + flash?]
Front-facing camera for videocalling
Magnetometer (digital compass)
Some links for the last few -
http://www.digitimes.com/news/a20090403PB200.html
http://www.macrumors.com/2009/04/08/...-possibilites/
http://www.macrumors.com/2009/04/16/...s-for-iphones/
http://www.macrumors.com/2009/04/07/...-focus-camera/
One of the macrumors.com links also mentions
video recording (found by delving into the OS 3.0 beta). Although the other things such as a 3.2MP camera and a magnetometer etc. would require a new hardware version, hopefully those of us with original/3G iPhones will still get the video recording feature with OS 3.0, seeing as AFAIK you can record video if you Jailbreak an iPhone.