Had a message from someone who moved to Melbourne and wanted a folding bicycle. I would still say a Dahon is good value, even though mine broke in half (and it is taking a long time to be fixed under warranty).
While looking up who sold folding bicycles in Melbourne, I noticed Dahon now sell a semi hard-sided case for transporting bikes by air, called the Airporter. There is also a "Carry Freedom Trailer" which turns the case into a trailer and packs inside the case with the bike for air travel. This is similar to the Bike Friday case Lynette Chiang used on her Cuba trip, but that had detachable wheels on the case to make it a trailer. Bike Friday offer cases and trailers.
Travel cases have wheels, so why doesn't someone make a semi-rigid travel case where these same wheels can be used to make the case a light duty bicycle trailer? The separate trailer ads to the complexity and cost: US$199.95 for the Dahon case and US$500 for the trailer.
On a 2004 Europe trip I used an small wheeled cabin bag as a bicycle trailer. The extended handle of the bag formed the tow bar (attached to the bicycle carrier with a stretch strap). This worked reasonably well for short distances, but the skate wheels of the bag were too small and hard. On pavement the wheels were noisy and on cobblestones the bag jumped around. Also the fabric of the bag was not very wear or water resistant.
In Marks and Spencer's at Cambridge I saw a shopping cart, with larger soft wheels and realized these would be better. Later at a display of suitcase production at the German Museum of Technology Berlin (Deutsches Technikmuseum) there was a semi-rigid wheeled bag with soft rubber wheels (about 90mm), which looked good. The semi rigid material is waterproof like a hard case, but flexible and light like a cloth bag (about half the weight of hard cases). The bags have zips covered with water resistant plastic gaskets.
Recently I bought a small carry on size bag made of the semi-rigid material, branded "Revelation" (made by by Antler) for under $80. Larger ones are around $100. These are not ideal as a trailer as they have the small hard skate wheels, the zips have a plastic seal only on the inside (more expensive ones have a seal on the outside as well covering the zip) and the extendable handle doesn't look strong enough to use as a tow-bar. As well the handle retracts into a hole in the case, making it less weatherproof and weaker.
So if someone out there wants to build the ideal bicycle trailer they would build a semi-rigid case with a weatherproof zip. The handle, retraction mechanism and wheels would be mounted in a grove molded in the underside of the case, to increase strength and water resistance. There would be large wheels with soft tires. The one retractable handle would be used to carry the case when closed, to guide it when extended as a wheeled bag and as the tow hitch when used as a trailer. The case would cost less than US$400 and weigh less than 10 kg.
There are equipment cases used for transporting video equipment with some of these features, but they weigh twice as much as the bicycle cases. You can even get a military specification mobile office on wheels, complete with desk, drawers and chair, which folds up into a travelling case. ;-)