I too use the Ds1307 in a few of my projects, most of which are based on an 18F4580. These are running at 40Mhz (10 Mhz xtal with OSC set to 40 and HS_PLL in the config), and on occasion these would either display 10:10 for time and date regardless of being either breadboarded or on a plug in module for the Easypic development board. I often found that changing the 37 khz crystal resolved the issue, or connecting a LED and suitable resistor between the SQW pin and GND so it pulsed at 1 sec intervals would somehow cause the chip to communicate to the PIC