I like the font, it is very clear to read it even when the text is very small and it looks very neat.
2 Answers
The iPhone 4 uses Helvetica Neue, a refined version of Helvetica [source].
Earlier iPhones use Helvetica.
Apple makes use of other typefaces too. Marker Felt was adopted for the iOS Notes app, although the latest version now uses a custom Apple typeface called 'Noteworthy'.
The most recent iPods (and the iPhone) use Helvetica as their UI font.
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2...though it's worth nothing that part of the reason the text is so clear, particularly at those small sizes, is that the pixel density of the phone itself is much higher than that of say, your desktop monitor.– Su'May 31, 2011 at 7:57