I've got a shield built, but never have gotten around to writing any code.
In spite of the partially complete MAX3100 project, I've come to the same conclusion as others - using an Atmel chip would be cheaper. The code has to be written on both ends, though. At least on the MAX3100 half the software is written already. Deciphering it may or may not be easier that writing your own; YMMV.
along with the 10uF caps needed
The MAX232 IC we all know and love is now commonly found in the 232A variant from most manufacturers, and the A only needs 0.1uF caps. Yeah, OK, it still needs 4 caps, but 0.1uF caps are small and cheap. I've had good luck with ceramic SMTs.