There is no performance issue with the 3G or 3GS running Flash.
But given that Apple have stated that they will not support it, I would not be holding my breath for it.
The basic issue is that supporting Flash applications on the iPhone undermines the App Store. As long as that is the case do not expect Flash support.
If things are limited to Flash video then there may be some leeway there, but otherwise if there is no way to control and/or monetize Flash applications (read games) I would assume it is just not going to happen.