The China Post has this transcript of an address by Thomas J. Christensen, Deputy Assistant Secretary of State, stating the official U.S. position on the security of Taiwan. (When you click on the link, you will be asked to install a language pack. Click cancel, you do not need the language pack, the Post is in English.)

In a nutshell, we want a democratic Taiwan, make vague noises against agression by China, but very much warn Taiwan not to do anything that might annoy China.