I don't know if you wander at all over to bikeforums.net but the tandem section over there is very helpful.

Good brands are-
Cannondale- great value for money, solid tandem builder, been doing them for ages.
Santana- great USA steel/AL/exotic metals tandem manufacturer. Can do custom.
Co-Motion- similar to Santana as above
Burley- they don't make tandems anymore but the second hand newer ones are good value.
Trek- they stopped making the T2000 and T1000 last year but a 2005 onwards models are very good value for money.
Calfee- the drool worthy carbon tandem manufacturer but $$$$
Da Vinci- these tandems usually have the indepedent coasting system (so the stoker can take breaks and vis versa), another good brand.

Those are the main BIG brand names. FYI although there are some tandems with Dura Ace, Ultegra level components are normally considered top level. Also you may want to invest in a Cane Creek Thudbuster seat post for the stoker, to make the bumps easier.