The SPD (Serial Presence Detect) is a small non-volatile memory chip placed on each SDRAM module. It is programmed at the factory when the memory module is built. It contains various module parameter information and is read by the BIOS on bootup (via a standard serial bus protocol - hence the "Serial"). It contains the module architecture (# of banks, # of rows, # of columns, etc) and information about what SDRAM timings the module supports. The BIOS uses this information to program the motherboard chipset - unless you have overridden those settings in the BIOS (which any self-respecting overclocker has done). So the memory module could say it's CL3, but if your BIOS allows it, you can override that and use CL2 timing.
There don't appear to be any timing parameters stored on the SPD that can't be overridden by a modern overclock-friendly BIOS (even on my trusty BH6).
So, IMHO an SPD programmer isn't necessary with a decent BIOS (unfortunately my laptop doesn't fit into that category, so I'm stuck with the CL3 timings programmed into my SODIMM SPD).
One final point - I finally dug up a datasheet for the SPD on my laptop memory module and it allows the bottom half of the SPD memory to be locked (write-protected) after programming. The bottom half of the SPD contains the above-mentioned info, and there is absolutely no way this lock can be reversed. From that point on the part becomes read-only and no programmer in the world can change that. I don't know for sure, but I would be willing to bet that the memory manufacturer performs a locking operation on each SPD after programming it. It is just one more write operation to the SPD that does the lock, so it wouldn't cost them anything to do it.
BTW - I figured this out recently because I bought a 512MB Crucial CL3 SODIMM module for my laptop and have been hoping to figure out how to get it to run at CL2 timings. Because the simpleton DELL BIOS doens't allow any timing tweaks I was hoping to re-program the memory SPD. It wouldn't be hard to build a programmer running off of the parallel port, but once I found out they can lock the SPD, I gave up. You would have to find an un-programmed SPD chip, pull the locked one off the module, and put the new one on. Not difficult if you have the right tools, but not many people have access to them (unfortunately including myself at the present time).
