Once a common sight, people in suits are now seldom seen riding bicycles in the UK. The only possible exceptions I can think of are some of the users of the London Cycle Hire and possibly some in Cambridge.
Recently, I have had cause to wear a suit more frequently. Despite this change of attire I have still had to get around, which for me means cycling. Generally I find formal wear to be a bit more restrictive than I like, although a decent suit makes a big difference. On the sorts of bike I used to ride, cycling in a suit wouldn't have been feasible, the rider posture combined with the restrictions on the arms and shoulders conferred by a suit jacket would be difficult, at least for me. I can imagine this situation would be made worse by a racing or touring bike, with the popularity of sport-oriented bikes, likely a contributing factor in the declining number of cyclists-in-suits.
The Brompton and Yuba Mundo, despite sporting reasonably upright riding positions, are both not ideal for cycling in a suit due to their exposed transmission. Trouser clips are a potential way to avoid the problems associated with an exposed transmission, but if it is essential that you make a good impression, a chain-guard or preferably a chain-case is the best way to ensure you reach your destination suitably smart. Cycling in a suit, just like almost all other cycling, is best done using a bike fitted with mudguards.
For this reason, the DL-1 has been my bike of choice for riding in formal wear. The upright posture prevents the suit jacket coming into conflict with the arms and shoulders, the chain-case prevents the trousers getting covered in chain filth and the generally relaxed feel of the bike prevents you from working up much of a sweat on your way to that important meeting. Plus it has a briefcase clip too. Now I just need a briefcase.