There was a press release about them:
These smaller Nucleos are called Nucleo-32
The boards use a 32pin MCU target. There are three signals, connecting the MCU to the ST-LINK/V2-1 MCU (SDIO, SWCLK and VCP RX), which aren't on the DIL pins. Those would have been useful on header pins. VCP RX would be good because it completes the USART (VCP Tx is available on the 2x15 pin DIL header).
Their are a bunch of useful documents, for example for the STM32F303 Nucleo-32 board:
The most useful seems to be "[url=http://www.st.com/st-web-ui/static/active/en/resource/technical/document/user_manual/DM00231744.pdf]UM1956 User manual. STM32 Nucleo-32 boards[/url]", which contains the schematics, and key to the board layout.
Like 'big Nucleo's" it has an ST-LINK/V2-1, which uses an STM32F103CBT6. This connects to the MCU's SWD pins, [i]and[/i] has a USART connection too. However, there isn't a SWD socket.
The schematic has the comment "closed for L021, L031,L433 open for F042,F031,F303" next to the crystal.
So it appears that there will be 6 MCU's on that PCB.
Their are pads for an external 32KiHz crystal (X1). The STM32F boards won't have that.
STM32Fxxx can be be driven by the HSI, or the ST-LINK/V2-1 'MCO' signal
When the crystal is [i]not[/i] connected, Arduino Nano-pins D7 and D8 are connected to the unused crystal oscillator pins, which can be used as normal GPIOs. So the STM32Fs have two more useful I/O pins than the STM32Ls.
I like the mbed-style upload on Nucleo's; they look like a small flash drive. Upload a binary-program using a file browser, command-line copy command, or any program (e.g. web browser) which can write a file, and upload is done. "Simples". This is so much simpler than using avrdude, ST-LINK-drivers, or texanes ST-LINK software. The STLINK/V2-1 implements a USB virtual comm port, so it might be as easy to talk to it from a host PC as an Arduino.
The STM32F3 is a very impressive part. It uses the same CPU as a Cortex-M4 (with hardware floating point) and much improved peripherals compared to the STM32F103. For example, the STM32F303 has 5Msps ADCs, compared to 1Msps for STM32F103, DACs, two analogue comparators, and a programmable gain Op Amp.