I believe this answers a few questions and more than a few complaints

Global Entry Members In Possession of a Global Entry Card May Utilize NEXUS and SENTRI Lanes Coming Into The United States.
http://www.globalentry.gov/howtoapply.html

(07/12/2011) U.S. Customs and Border Protection (CBP) will begin issuing Global Entry radio frequency identification (RFID) cards on July 12, 2011 to qualified U.S. citizens, U.S. Lawful Permanent Residents (LPRs) and Mexican citizens (not already in possession of a SENTRI card) who are approved for membership in Global Entry. There is no additional cost for the RFID card as it will be included in the application fee. Global Entry members who were approved for the program prior to July 12, 2011 and wish to obtain a RFID card may request one via their Global Online Enrollment System (GOES) account for a $15 fee.

Global Entry cards are Western Hemisphere Travel Initiative compliant documents for land and sea travel. Additionally, these cards will be valid for use at the SENTRI and NEXUS lanes coming into the United States. However, Global Entry cards will not be valid for entry into Canada via the NEXUS lanes and kiosks; and members must use a passport or U.S. LPR card at the Global Entry kiosks. Please note that as of the above date, members can use any CBP issued Trusted Traveler Program card when traveling inbound into the United States via the SENTRI or NEXUS lanes.

Any Global Entry member who wishes to drive their vehicle in the SENTRI lane must register their vehicle and have a 7-point inspection conducted at a SENTRI Enrollment Center. For more information on locations of SENTRI Enrollment Centers as well program requirements, please go to www.sentri.gov.