Does anyone have any recommendations on an embedded platform that would be able to handle RS232 at 921600 baud and log it to a microSD card or something similar?
According to my reading of the datasheet, an Atmel ATmega running at 20MHz (the max) can run its UART at up to 2222222 baud rate (~2.2Mbps), more than double the speed you need.
(BAUD = Fosc / (8 * UBBRn + 1)) with Fosc = 20000000 (20MHz) and UBBRn = 0 (the minimum)
So the UART reading part should be no problem. The only question now becomes the writing to the SD card. You can do that over SPI, which is separate from the UART, so you can run them both in parallel with little overhead. According to this site, when running an ATmega at 8MHz (thus 4MHz SPI clock), you can write to a SD card at up to 140kB/sec. You only need ~128kB/sec, and ATmega's can run up to 20MHz (thus 10MHz SPI clock), so there shouldn't be a problem there.
The Parallax Propeller will do serial full-duplex at 3 Mbps .
Of course, it will run out of memory in a hurry unless you put the data somewhere. Also, since the serial interface is run in one of the internal processor cores, in software, it is basically infinitely adjustable to any frequency between 0 and ~3 Mbps.
Here is the serial interface module
Also, there is a SD-Card module that supports FAT16/32 cards
I'm not sure if it can keep up with the serial interface though.
There are a lot of other useful pre-written object in the object exchange too.
Basically, the Propeller is awesome.
For those of you who aren't familiar, it's an 8-Core 32 bit microprocessor, with both a interpreted high level language, and a custom assembly language for speed-critical stuff.
I've found that it's possible to get away without actually having to use assembly at all, since there are so many available libraries. You mostly just need to write glue-logic to link them together.