lol... I also have friends who do that when they come into my office... And like you, I try to keep only the sturdiest of my mementos out because it's happened quite a few times that some of the more fragile ones got damaged... One that attracts a lot of attention is a bank of huge rectifiers with heatsinks that I recuperated from a dismantled power plant project... The great handiwork makes a nice conversation piece and the technos _will_ touch it, but it's heavy enough that they will usually refrain from casually picking it up ;)
Now come to think of it, I admit to this klepto tendency, but instead of picking up things I tend to jingle the keys in my pocket :D
@Old_Faithful - Thanks for the explanation. So what you mean to say is that this lady (she's Tina, I presume) has a 'Howard Hughes' kind of syndrome (I remember Leonardo DiCaprio in The Aviator obsessively scrubbing his hands with soap and water after shaking them with others.)
Along similar lines, I have an old Matchbox toy (a small military jeep with an attached trailer and a tiny driver) kept on my CPU. Everyone who comes over to my desk to have a short chat ends up picking it, driving it a little on the cubicle wall and then inadvertently dropping it on the floor 3 feet below. This has happened at least half a dozen time and the jeep's still intact, a testament to Matchbox's superior plastics and build quality :D