I could not recommend against using the Dahon BioLogic case more strenuously.
The catch at the bottom of the case is woefully inadequate for holding the handset between the two sides of the shell. I had my phone fall out when riding over a moderately uneven patch of road, subsequently having it fall upon said asphalt and smashing the screen.
The case is also incredibly cheap flimsy plastic, as is the handle bar attachment loop.
The only case I've found to be suitable to hold my phone securely was the GoRide (
http://bicio.com/), but it provides no protection of the screen, and they're yet to release an iPhone 4 shell.