Much like the widely-publicized i Phone 4 hardware issues, it's hard to believe that an issue as widespread as i OS 4's terrible performance on the i Phone 3G could have escaped the notice of Apple's testers.
The main appeal of Apple's products, and the main point of differentiation it's pushed in its advertising, is the high quality and ease of use of both Apple's hardware and software.
This was an interesting move considering the i Phone 3G has the same CPU and RAM as the first-gen i Phone.
If i OS 4's performance on the i Phone 3G is any indicator, Apple was right to leave the original i Phone by the wayside, and perhaps should have left the i Phone 3G out as well.
Under 3.1.3, I had hardly any slowdown issues at all; after the "upgrade" to i OS 4, I couldn't even run the i Pod and Maps apps at the same time without having my music stutter like a record player on a garbage truck.