Thanks for all the replies, which were very useful! I've had & loved my Dahon Vitesse D7HG for over a year, now.

Distillation of the advice, with my own additions:

• Consider other brands, particularly b/c a lot of cheap bikes have become available. However, I still recommend Dahon having ridden it.
• Buy new. My attempts to find & buy used bikes just introduced me to a lot of crazy people who kept changing the deal, and one sane person who simply had a very old bike. You'll have too much riding on a complicated folder to trust an old bike.
• Definitely test ride it before finalizing your decision. Mt. Airy Bikes was a great suggestion and friendly people, even though I bought from someone closer to home. On that note, buying close to your workplace might make more sense; commuting home and then going to the bike shop before closing time is still difficult.
• MARC Train commuters: Except maybe for some of the Brompton's and perhaps the Dahon Mu Uno, I think you'll only be able to fit your bike on the two-tiered cars in the lower luggage rack that's next to a seat, not the ones by the door. Single-tier cars (Camden Line!) have no luggage areas and no space! Taser, above, suggested putting it in the vestibule. I'd be amazed if that didn't get you grouchy comments from the conductors, but if you have to, do it. On the Amtrak cars, stick your bike in the coat-rack closets; you'd have to lie your bike down to put it in the actual luggage rack.
• Dahon's two-legged kickstand scrapes when you go around corners & over curbs, grr, and isn't strictly necessary for sitting your bike down, but I still find it worthwhile & friendly to my commute.
• The ability to roll your bike while folded is important, as stated above! The Bromptons look insanely friendly in this regard; the Dahon's pretty good. I've heard complaints about inability to do this with cheaper bikes.
• 16 or 20"? I don't think 16" would make the bike any thinner, so go 20", the height's not what's keeping it out of luggage racks.

Thanks for all the advice, and I hope this summation helps someone else