Good point on the misspelled address, which happens more than you think. I forgot about that one.
As for being able to delete the cache, I don't know if it is necessary to delete the entire cache, as I believe that would clear all of your previous recipients. I just want to selectively remove the ones that aren't relevant anymore, i.e. the ones that have changed, or are misspelled, etc. This can be done in OS X Mail fairly easily, and with with iPhone OS 3.0, the capability should be there, so I don't know why Apple wouldn't make it available. I'm more hopeful than not, but it will require patience.
If anyone can think of other reasons why this would be a helpful feature, please respond and list them here. I've already submitted an enhancement request to Apple. Why not do the same?