I will take this bike in to the shop tomorrow and have them look at the loose cup. Thanks a lot to everyone for lots of ideas and advice, it is appreciated.

I am wondering if anyone would care to speculate about how the cup got loose? It is the headset that the bike came with (my girlfriend bought it used), and before I overhauled it everything seemed fine. When I cleaned the parts I didn't attempt to remove the lower cup or the upper race (i.e. the parts that are press fit into the head tube). However when I was taking it apart I very stupidly had the frame in the repair stand, and when I gave the headset that final turn the fork went crashing to the floor ... then I did a nice amount of cursing. I was worried about damage to the fork but it seemed fine, but could the force of the fork falling have bumped the cup out of place? If so I am double-kicking myself for not being more careful.

Thanks again for everyone's help