Dual core Pentiums appear to be incompatible with existing 915 and 925 socket 775 motherboards. There is socket compatibility, but if you block one in then it will not work, apparently. Instead, motherboard manufacturers will have to redesign the motherboard in order to make dual core 8xx CPUs work in existing boards. But the chances of this are pretty low.
Early adopters of socket 775 will have to buy a new motherboard based on the 945 or 955 chipset in order to make their new dual core CPU work.
Many won't be happy to replace their motherboards just months after they bought them but they really don't have any other choice.