Great thread with some awesome info. I went over my Burley Duet today and here are her specs:

SN: C1613
140mm rear spacing frame
Front eccentric bottom bracket width 175mm square taper
Rear bottom bracket width 180mm square taper
Sugino square taper crankset Front 175mm (40), Rear Triple 170mm (54-44-28)
Shimano Deore XT Rear Derailleur 8-SIS
Shimano Deore LX Front Derialleur
Rear Wheel is anodized Sun Chinook rim, 48h Phil Wood hub and Sachs 7-spd 12-28 cassette (drag brake capable)
Spare Rear Wheel polished Sun CRT16II rim, 48h Sansin hub and Suntour cassette 12-28 (drag brake capable)
Front Wheel polished Sun CRT16II 48h Sansin hub 140mm
Bike is currently setup with flat bars and mountain bike Shimano XT shifters

Now that I've disclosed the specs hopefully this will help with my upgrade question. I would really like to get more cassette gearing and go up to at least a 9-spd. I found a really nice take off 700C wheel set for sale. It has White Industries M16 40h hubs and Sun ME14A rims with machined side walls and black anodize. These babies are sweet and can take cassettes up to a 10-spd. Now comes the crank question; can I use my current 7-spd crank-set with a 10-spd cassette? Aren't the chain and chain-rings thinner on a 10-spd? And if they won't work what should I use for a crank-set and bottom bracket setup? I'd really like to have that large 54 chain-ring size.