Also: http://www.iaadp.org/usdot-may2008-airline-guidance.html

    .....The only situation in which the rule contemplates that a service animal would not be permitted to accompany its user at his or her seat is where the animal blocks a space that, per FAA or applicable foreign government safety regulations, must remain unobstructed (e.g., an aisle, access to an emergency exit) AND the passenger and animal cannot be moved to another location where such a blockage does not occur. In such a situation, the carrier should first talk with other passengers to find a seat location where the service animal and its user can be agreeably accommodated (e.g., by finding a passenger who is willing to share foot space with the animal). The fact that a service animal may need to use a reasonable portion of an adjacent seat's foot space—that does not deny another passenger effective use of the space for his or her feet—is not, however, an adequate reason for the carrier to refuse to permit the animal to accompany its user at his or her seat. Only if no other alternative is available should the carrier discuss less desirable options concerning the transportation of the service animal with the passenger traveling with the animal, such as traveling on a later flight with more room or carrying the animal in the cargo compartment. As indicated above, airlines may not charge passengers with disabilities for services required by Part 382, including transporting their oversized service animals in the cargo compartment.

In modifying this paragraph in the guidance, we deleted the phrase concerning the potential purchase of a second seat, since there are probably no circumstances under which this would happen. If a flight is totally filled, there would not be any seat available to buy. If the flight had even one middle seat unoccupied, someone with a service animal could be seated next to the vacant seat, and it is likely that even a large animal could use some of the floor space of the vacant seat, making any further purchase unnecessary. Of course, service animals generally sit on the floor, so it is unlikely that a service animal would ever actually occupy a separate seat. ....

Of course mine could also sit in that extra seat with a properly-rated seatbelt harness for takeoff and landing, at those times people are required to wear their belts?

~Susan