Vernier Sensors will make their adapter for the NXT avaiable in October. You can find this adapter from LEGO Education.
With this adapter, any of Vernier's 40+ analog (BTA) sensors can be incorporated with the NXT. Vernier sensors can be purchased at http://www.vernier.com/.
Update: Use the following link to view more details of the adapter: http://www.legoeducation.com/store/detail.aspx?CategoryID=159&by=9&ID=1442&c=1&t=0&l=0 (If the above link causes trouble, the item number of adapter is W991377 on the www.legoeducation.com United States site.)