1. Why iPhone?
This has nothing to do with "rich" or "fanboy" or whatever. Fact is, that the iPhone has become a global marketplayer in the world with a very huge Appstore market. In fact, I think that BookCrossing already missed a real opportunity, as it should have been visible at that Appstore from the very beginning. But better late, then never.
And although a plaintext / webbased version of the site would do the thing on an iPhone too, it would not be the same usability (except if you build an extra site with webkit, which offers the same usability. But that has to be developed too, where as plaintext is "just" a CSS thingy).
The other things is, Webbased / Plaintext versions always needs an internet connection. That's going be expensive, when you are abroad. With an App, and hopefully the programmer will keep that in mind, you can just release at the spot, take your pictuere, and everything will be queued until you have internet access again. You want feel the difference then ;)
2. Only iPhone?
Of course not. But the iPhone market is the biggest at the moment and it seems that going from a functional iPhone App to an Andriod version is not that difficult anymore. So once an iPhone App is proofed and running, the andriod version (c)(sh)ould be follow quite soon.
3. Webbased version / Plaintext
Something we do need as there are still (*veg* ;)) other devices and slow computers out there. And as said before: the biggest disadvantage of webbased solutions is, that you always need an internet connection to use that solution. Abroad you may not able to do this (at least not at expensive costs). So specific apps for specific smartphone do make sense IMHO (well as long as they also can operate in an offline mode).
This would be something nice, not only for Smartphones but in overal. (Experienced) BookCrossers could create all sort of things based on BookCrossing. Games, specific challenges, utilities, statistices, etc.
Still, BookCrossing.com itself should offer their app - at least to get some pennies to pay off the cost of this wonderful site...
As said, just my point of view I'd like to share :)
*removed the off topic word :)