The 9300 Curve 3G is nearly identical to the current 85xx series Curve devices from AT&T and T-Mobile. It has a 2 megapixel camera, 802.11b/g/n Wi-Fi, GPS, Bluetooth 2.1, and support for microSD cards up to 32GB. It is smallish, uses an optical trackpad for on-screen navigation, and is the entry-level BlackBerry.
The first key feature of the new 9300 is that it supports 3G for the first time on GSM-based networks. The 85xx series Curves were stuck on 2.5G EDGE for wireless data. The 9300 comes in two 3G radio configurations, 800/850/1900/2100MHz for AT&T and 900/1700/2100MHz for T-Mobile. T-Mobile confirmed that the 9300 Curve 3G will be available in the "coming weeks."
The other key feature is that the 9300 can be upgraded to BlackBerry 6, RIM's new operating system announced on August 3. The 9800 Torch will be the first RIM device to launch with BlackBerry 6. RIM announced that BlackBerry 6 will also be available to the current BlackBerry Bold 9700, Bold 9650 and Pearl 3G handsets within the next few months.
(Curiously missing from the list of handsets that can run BlackBerry 6 are RIM's own touch-based Storm and Storm 2 handsets. Why neither can run BlackBerry 6 is a mystery that RIM has yet to clear up.)
BlackBerry 6 includes a "fresh but familiar" redesign that has a heavily revised home screen, offers better social networking integration, improved alerts, fewer menus, and a more finger-friendly touch user interface.
T-Mobile didn't say when the 9300 Curve 3G will be available, nor exactly how much it will cost. I'd expect a fairly low price point, however, as T-Mobile (and AT&T) both already sell the more advanced Bold 9700.
Though the 9300 Curve 3G will support AT&T's 3G bands, AT&T hasn't officially announced that it will sell the device.
You can read more about the Curve 9300 on RIM's web site here.