Not only will DDR3 initially be slower than DDR2 memory, it will also costs a lot more.
FUD Zilla heard the price of a 2x 1GB kit of DDR3 memory will cost about $600. That's three times as much as the price of a good DDR2 memory kit.
The production is still ramping up and the early samples are expensive and the timings are bad. There might be more samples in late May as they are almost non existent today but you don't have to worry as Intel P35 will wait for Computex to officially resurface.
I think that you will see DDR3 kits here and there but it wont be worthy for a while as it will be extremely expensive and you wont get the desired performance out of it. The only good thing is that it will work will less power and is likely to be overclockable.