I have been running mine at about 28psi, tubeless, but I've been thinking that I can get away with low 20's, because I didn't smack the rims all last summer...oh, wait, this is the Road Bike forum...well, then.

Sheldon wrote about it. The takeaway is that you can easily overinflate your tires and they'll start to bounce on little bumps (like seal coat) and you'll see a reduction in speed because you are constantly losing a little traction. A more detailed workup for you engineers can be found here and the most detailed CRR study can be found here. Keep in mind that the CRR data collected here were taken from a smooth roller so the real world issue of tire pressure never really comes up.

Oh, and unless you want to work on your French, you should select the English option when you are at rouesartisanales. Bottom line is, unless you are on 19s or are a really big boy (wooky class), then 120 psi is too high. 100 psi is probably too high for you lightweights.