I have used other phones with virtual keyboards. Most of these phones have been Windows Mobile based. One of the complaints that I have always had was when I had to press other keys after the phone call. You know what I mean, you need to press 1 or 0 to reach someone else of maybe press pound.
In most virtual keyboards, the screen is dimmed after the phone is connected to save power. This is true of the iPhone as well but since the iPhone senses motion it realizes that you have moved it from your ear, so the display lights up again so you can type.
This is one of the reason that the iPhone is easily the best phone I have ever owned.
This post was written on my iPhone.