Someone on a mailing list I'm on was asking about small o/s's. Someone else suggested "contiki".
It's a nice small O/S with a user interface. Might be the right thing for a small handheld, who knows. I like the look of it.
Someone else mentioned UniFLEX. That archive is amazing becuase there it's written for the 6809, has FORTRAN, Cobol, Pascal and C. Wow.
And of course, others mentioned MicroC/OS-II. . Certainly a good choice for real-time. I've used it and liked it.