4.3" screen (the Fusion edit: make that "Infuse" is 4.5") versus 3.5" screen is a massive difference.
Apple pushes Facetime yet Samsung SGII has front camera with 2MP 1920x1080 versus the iPhone at VGA 640x480 (.3MP)
Vlingo is pretty darn good too.
Although Android has to fix stuff like this http://i.imgur.com/mGwxD.jpg
It is odd that there isn't any ready-to-go Samsung app for it (I have an SGII) or at least I didn't see one yet.
It's not a big surprise though since not all phones which use the Android OS have a front facing camera so it would be up to the individual phone manufacturers to add an app if their phone has a front camera.
If it's a smartphone I would say it's main purpose is to view websites and use apps such as Facetime or equivalent which would be uncomfortable on a small fuzzy screen.
Some day we get the roll-up displays and have scroll like phones with large folding/roll-up screens.
My claim is: 3.5 inch screens are a good compromise, allowing for comfortable browsing, reading and writing. The additional screen space would never be worth the additional bulk (also: uncomfortable one hand use) for me.
That's all about personal preference.
Someone with an iPhone would be used to the "hand feel" of a 4" (101mm) iPhone if the screen bezel were thinner.