Wouldn't it make more sense to charge forum owners for the plug-in instead of end-users for the app? The reason I say this is that I doubt there is going to be a flood of iphone users paying $3 to browse a forum they can already browse with safari. However, if it were free, they might just grab it. As a forum owner I would actually prefer paying for it instead of my users. I don't want to put the time into adding it to my forum,testing, etc, with the thought that not many will probably use it.
Just my 2 cents and I could always be wrong.
-vissa