Daring Fireball: Hacking the iPhone Notes App for the Admittedly Nit-Picky Purpose of Changing the Text Font to Helvetica:
I can’t recommend doing this. Binary files are fragile; make a mistake while editing and it’s likely, very likely, the application will no longer work properly. Plus, any changes you make to MobileNotes today are going to be overwritten before the end of the week, as Apple has already indicated that the next iPhone software update is imminent.
On the other hand, if you have the urge to hack around on your iPhone, you might as well do so now, as I strongly suspect that this week’s imminent iPhone software update is going to render inoperable the existing ways of hacking/jailbreaking the iPhone. Notice how no one’s yet figured out how to install or modify the software on the iPod Touch? Whatever Apple’s doing on the Touch in this regard, I expect them to begin doing on the iPhone this week. (I’d love to be proven wrong.)