Hexus.net vaguely claimed that Intel's Core duo may have support for 64-bit processing. The report is poorly written so is difficult to gather much from it, however, several Mac websites have started reporting it, so it is now linked here for interest sake. It appears that even if the Core Duo had hidden 64-bit capabilities, it is not active on shipping models, making the point somewhat moot.
It appears the basis of this rumor is that Intel's upcoming Sossaman processor is 64-bit, however, a recent press release specifically notes that Sossaman is a 32-bit processor.
The new dual-core Intel Xeon processor LV, codenamed "Sossaman," is designed on Intel's 65-nanometer technology manufacturing process. Sossaman, based on the Intel's next-generation, power-optimized micro-architecture, will offer dramatically improved performance and greatly reduced power consumption. The brand new 32-bit dual-core Intel Xeon LV processor with 2MB of L2 cache running at 2 GHz is based on the existing Pentium M architecture and offered power-saving features with Demand Based Switching, and Enhanced Speedstep Technology (EIST).