All 58 iPhone fonts now available as of 1.1 (Feb. 5/10). Here's a handy list of current iPhone fonts -- use their full name, like "TimesNewRomanPS-BoldMT" to reference them:

[1] AppleGothic
[2] HiraKakuProN-W6
[3] HiraKakuProN-W3
[4] ArialUnicodeMS
[5] STHeitiK-Medium
[6] STHeitiK-Light
[7] DBLCDTempBlack
[8] Helvetica-Oblique
[9] Helvetica-BoldOblique
[10] Helvetica
[11] Helvetica-Bold
[12] MarkerFelt-Thin
[13] TimesNewRomanPSMT
[14] TimesNewRomanPS-BoldMT
[15] TimesNewRomanPS-BoldItalicMT
[16] TimesNewRomanPS-ItalicMT
[17] Verdana-Bold
[18] Verdana-BoldItalic
[19] Verdana
[20] Verdana-Italic
[21] Georgia-Bold
[22] Georgia
[23] Georgia-BoldItalic
[24] Georgia-Italic
[25] ArialRoundedMTBold
[26] TrebuchetMS-Italic
[27] TrebuchetMS
[28] Trebuchet-BoldItalic
[29] TrebuchetMS-Bold
[30] STHeitiTC-Light
[31] STHeitiTC-Medium
[32] GeezaPro-Bold
[33] GeezaPro
[34] Courier
[35] Courier-BoldOblique
[36] Courier-Oblique
[37] Courier-Bold
[38] ArialMT
[39] Arial-BoldMT
[40] Arial-BoldItalicMT
[41] Arial-ItalicMT
[42] STHeitiJ-Medium
[43] STHeitiJ-Light
[44] ArialHebrew
[45] ArialHebrew-Bold
[46] CourierNewPS-BoldMT
[47] CourierNewPS-ItalicMT
[48] CourierNewPS-BoldItalicMT
[49] CourierNewPSMT
[50] Zapfino
[51] AmericanTypewriter
[52] AmericanTypewriter-Bold
[53] STHeitiSC-Medium
[54] STHeitiSC-Light
[55] HelveticaNeue
[56] HelveticaNeue-Bold
[57] Thonburi-Bold
[58] Thonburi

It looks like they added some fonts in later OS updates; I believe my list was from OS 3.x. Here's a list from OS 2.1, which is a few fonts shorter:

http://ajnaware.wordpress.com/2008/10/24/list-of-fonts-available-on-the-iphone/