I might as well be the first to say it because someone will, here we go 3G iPhone. Thats not what I think but thought I would get that out there so we can look at something a bit more realistic.
I reckon if this "reliable source" is right it will be the same iPhone with just some different applications and or additions to applications. The iPhone will remain the same iPhone that is in the US but over here it will probably be released with MMS, thats a definate possibility and something I dont think is unreasonable. Also it will possibly get bluetooth filesharing and someway of putting on customisable ringtones.
Im no expert on the US mobile phone market, but in the UK that is definately the three main things that people will be after I would have guessed. The rest of the iPhone I would say will remain unchanged.
Who knows what Apple will do, we will find out nearer the time (better be soon I am itching to get one!!!!)
Edit: Not first to say 3G, bet me to it, but I told you so someone would put that one out first!!!!