Well, we did it. After a lot of helpful inputs from all of you on this forum, we made our decision and picked up our "new" 2007 Co Motion Speedster this afternoon. We started our search looking at older (and less expensive) used bikes, but just kept edging up the scale. We are so sure we are going to love "tandeming", we wanted to make sure we got something we wouldn't be replacing in a year or two.
The bike was originally purchased as a frame only from Co Motion and built-up by our LBS. It has all Ultegra components (including rear derailleur), the Avid rear disk brake, a Chris King Headset and Sun rims/White Industries hubs (the bike had Rolf Primas, but we traded to save $$'s).
Now all we need is some nice weather and a deserted stretch of road to figure everything out (keeping in mind that the stoker is NEVER wrong).