Citrus is self-thinning. Fruit falls off if more is started than
the tree can support. Other causes of fruit-drop include improper
watering (too much or too little), poor drainage, lack of zinc, and
insect damage

Thanks for your input David. The tree is rather small, only about
three feet tall. Perhaps it's not ready to form fruit yet? I'm also
wondering about the watering. What kind of watering schedule do citrus,
and lemons in particular, like? I water the tree along with the rest
of my patio pots, which seem to do fine with a good soaking every two
or three days.

-Fleemo