+1 on looking for a good, used Santana or Co-Motion. I have owned a Santana Rio, and currently own a Co-Motion Mocha (both with 26" wheels) and they are both good bikes.
I agree that a Co-Motion Periscope might be a good option, too. These seem to come up used fairly regularly, and in good shape, as many folks buy them to ride with their kids and then sell them when the kids "graduate" to single bikes. They work perfectly fine as adult bikes, too, and are very adjustable. By the way, if you find a Co-Motion that you like at a good price, bu the color doesn't work, Co-Motion does repaints for somewhere around $600+ IIRC. There's a nice-looking Periscope on this page for $2000:
Tandem Bicycles For Sale, Tandem Bike Components For Sale
The Hokitata line of bikes that TandemsEast sells also seems to be a very good value. I've seen them in person at TandemsEast events and wouldn't hesitate to buy one. They can also be custom-ordered, I believe (I've always guessed that Rodriquez makes the frames for TandemsEast, but never have confirmed this).
Also consider Cannondale tandems. These can be very good values used.
Personally, I'm not keen on the 650 wheels replacing 26" wheels. Very similar in size, yes, but tires are still much less available. But that's another thread! :-)