Shopping for embedded OS + platform

I am designing a small device that will sit on a telephone line and 'split'
telephony data and encoded audio, passing it over USB to a nearby PC.

My needs are:
- ~128-256K RAM I/O buffer
- USB interface, must supply power
- LED output
- Reset button
- Debugging interface, assume RS232
- Interface to telephony chips, believe this will be I2C, not sure yet ...
- Open source tool chain + OS, GNU is ideal
- Low hurdles to entry in terms of both cost and time. I am a windows
software dude with minimal Linux experience and want to get up to speed in a
hurry.

My "wants" are:
- Prefer being able to code in C/C++
- Prefer but perhaps do not require multithreaded OS
- Boot from ROM (could be FLASH instead)
- Cmd line shell for debugging

Do not need or desire:
- File system
- Heap
- UI except LEDs + reset button
- Ethernet or other net adapter

I am looking at embedded Linuces but the "super small" one seems to take 32
MB of RAM to boot, which is massive overkill for my purposes.

Is there a small-n-simple, low cost solution for this situation??

RDeW

riley dot dewiley at gmail dot com