I wonder what kind of buildings are less susceptible to these kinds
>of problems. And is there a good way to test your data centre
>to come up with some kind of vulnerability rating?
>Would a Faraday cage be sufficient to protect against cosmic ray >bit-flipping
>and how could you retrofit a Faraday cage onto a rack or two of gear?
Unfortunately, you're in the wrong part of the spectrum there. Shielding against EMI to protect against solar flares is like ... well, it's like writing IPv6 ACLs to protect your IPv4 network. You're looking in the wrong place.
Solar flares have all sorts of effects on earth, including every piece of the EM spectrum (you get ionization, which has secondary effects like interfering with the amateur bands so ham operators can't click at each other very well, and induction of current in long wires because of the magnetic effects), but the nasty effects from our point of view are likely to be in piles of hard and soft x-rays and proton storms.
So to answer your questions directly: less susceptible building would be buildings which are constructed of hundred-foot thick solid lead walls under a mile of rock in a salt mine under a mountain in Wyoming.
And "no," Faraday cages don't protect against cosmic rays (or most of the other kinds of radiation that a solar flare emits directly).
Of course, IANAPhysicist, but I did play one in college for a while.
On the other hand, another effect of solar flares is UV radiation, so a good pair of sunglasses and some high-SPF sunblock would be good to have, plus make you look less like a nerd. Unless you use that zinc stuff on your nose, in which case you look more like a nerd. YMMV.
jms