Whatever anybody says, I'm sure Apple made a big deal of the fact that developers would have to develop apps for a single resolution only. This was aimed towards mobile developers, who had been having to develop apps for various resolutions. Thus, the chance of them introducing a higher resolution would be minimal, in my opinion.
To be completely honest, the current resolution is absolutely fine, and ever since November 11th 2007, I've never found any need to have a higher resolution on the iPhone, and I'm a bit of a resolution freak!