Patgadget: I recently tracked this down as a result of other mysterious behavior (sketches not including Serial function still using 128+ bytes for the serial input buffer, which they didn't in 11 or 12.)
In 12 and earlier, Wiring_serial.cpp.o and the rest of the arduino core library are assembled into a "core library file" core.a, which is then searched during the final link for needed functions.
In 13 and later, new compiler switches are introduced that permit unused functions and data to be omitted from the finished sketch even if they weren't in a searchable library. This was THOUGHT to do away with the need for building core.a as a separate step, but apparently there are some subtle differences.
Previously when you used your own serial code, you didn't reference the Serial.xxx functions, and the Wiring_serial.cpp.o module was not included in your sketch (from the core.a library) at all. With 13, it is linked in explicitly and expected that most of it will go away as being unreferenced, but some global code like the ISR itself has to be included (called by the hardware, you know), and that references the serial buffering, and ...
I've requested that the core.a code be put back as well as the new stripping feature (assuming that they work together ok.) We'll see what happens.
The relevant change was http://code.google.com/p/arduino/source/detail?r=520
It looks pretty easy to fix on your own, if you want to build from sources.