Broadcom's "BCM2157 3G HSDPA 'Android' Baseband" chipset, for example, provides everything a modern smartphone builder needs: a dual core ARM processor, Bluetooth, GPS, support for up to a 5-megapixel camera, support for capacitive HVGA (320x480 like iPhone 3GS) or or WQVGA (~240x400) displays.
The chipset will work on AT&T (T) and T-Mobile's 3G networks in the US and on global GSM providers.
An unsubsidized $100 smartphone will enable more purchases of service plans without contract, and arguably increase competition between mobile service providers. So it is possible that data plan prices could drop as well.
At the very least, m,ore consumers might start using Wi-Fi-only data connections. In one sense that is not a direct driver of data plan sales. On the other hand, use of Wi-Fi connections will create a new habit that indirectly drives more data plan sales.