While I bet there is a pd object to access serial data, one thing you might consider is converting your serial data into OSC messages. I've done this with an Atmel Atmega16 sending serial data to a computer, then I wrote a C program that converted this data into OSC packets that were sent over a socket connection (or over a network with UDP packets), and I've accessed this data with PD's OSC object.

The thing about making OSC messages is that you could also use this with other software.

If you are interested in the code I have written or talking about this further please feel free to email me further.

-Alex Norman