On the iPhone 4, the system fonts are Helvetica NeueUI, which is font02.ttf (normal) and font03.ttf (bold) in HelveticaNeue.ttc, another font collection in addition to the Helvetica.ttc we've already been modifying for ios4 and iPad 3.2.x.
Attached are the Droid Sans equivalents for iPhone4 users ONLY! Drop them into the /System/Library/Fonts/Cache folder after first making backups!
* Attached File: iPhone4_DroidSans.zip
* Attached File: DroidSans 008.png
* Attached File: DroidSans 005.png
* Attached File: DroidSans 006.png
* Attached File: DroidSans 007.png
Props to bill90039