"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Apple Lists iPhone OS 4 Compatibility, Excludes Original iPhone and 1st Gen iPod Touch
iPhone OS 4 will work with iPhone 3G, iPhone 3GS, and the second- and third-generation iPod touch this summer, and with iPad in the fall. Not all features are compatible with all devices. For example, multitasking is available only with iPhone 3GS and the third-generation iPod touch (32GB and 64GB models from late 2009).The original iPhone and 1st Generation iPod touch are excluded from iPhone OS 4 compatibility, while the 3G and 2nd Generation iPod Touch exclude multitasking abilities. Customers will have to have an iPhone 3GS, 3rd Generation iPod Touch, or iPad to access the full feature set including multitasking.
iPhone OS 4 is due this summer. To gauge how many of our readers will be able to take advantage of the new OS, we've created this simple poll asking the latest iPhone or iPod Touch hardware you own. Note that we've purposefully excluded the iPad in the interest of simplicity:
Detailed differences between models are listed here: iPhone, iPod Touch.