Some of the info on this thread is incorrect.
Fact: DDR2 800mhz RAM WILL work in a DDR2 667mhz slot JUST FINE. They are pin compatible. The only difference is that the DDR2 800 RAM is tested and rated for a higher speed. The RAM speed in a computer is determined by the motherboard chipset though, not the RAM. So running it at 667mhz is perfectly fine. (i.e. driving a car rated for 70mph at 60mph is fine. But if your car is only tested to go 70mph, best not run it at 80!)
Once again, please see my second point about how there are things OTHER than just the rated memory speeds that affect compatibility. I have no problem believing that your testing yielded the results that you posted - but this is not because of the fact that it is ddr2 800.The TESTED FACTS are:
PC2-6400 WORKED in Merom SR MBPs
PC2-6400 DOES NOT WORK in Penryn SR MBPs.
I understand that you are advising people to buy RAM guaranteed to work with their Mac, and I agree. Just wanted to state that in this case, 667 and 800 are not compatible.
yup - that's why just pay like 5 to 10 percent more and grab some RAM from a place that has good results in the mac community. I've been burned by trying to save a few bucks by getting cheap RAM elsewhere and I def regretted that.and RAM that is slightly off-standard gets gonged.
Most likely because the newer laptop run a different CAS/tRCD/tRPD/tRAS setting that your memory SODIMM can't handle. Has nothing to do w/ the fact that it's DDR2 800. Apple is pretty mysterious about what their timings settings are or when they decide to change them. moral of the story: just get RAM from a seller that guarantees it to work in the mac.Then explain why the ram works just fine in a pre-penryn mbp and not in a penryn mbp? It's clearly some change in the laptop itself, not the RAM.
Edit: I went to the mfg's site before school and viewed the data sheets on the RAM. the voltage, cas timing, etc, all identical. I'll try to post links.