Some sites have published info about iOS 4.2 beta which has been made available to developers for testing before release in November. According to this, the iPad will have the following virtual keyboards at that time:
English, English (UK), Arabic, Bulgarian, Catalan, Cherokee, S. Chinese (handwriting, pinyin, stroke), T. Chinese (handwriting, pinyin, cangjie, stroke, zhuyin), Croatian, Czech, Danish, Dutch, Estonian, Finnish, French, French (Canada), French (Switzerland), German (Germany), German (Switzerland), Greek, Hebrew, Hungarian, Icelandic, Indonesian, Italian, Japanese (romaji, 50On ), Korean, Latvian, Lithuanian, Macedonian, Malay, Norwegian, Polish, Portuguese (Brazil), Portuguese (Portugal), Romanian, Russian, Serbian (Cyrillic), Serbian (Latin), Slovak, Spanish, Swedish, Thai, Tibetan, Turkish, Ukrainian, Vietnamese.
If this list is correct, some keyboards available in OS X will still be missing: Devanagari, Gujarati, Gurmukhi, Tamil.
In addition, some fonts that come with current iOS 4 (but not with OS X) will not have keyboards: Bengali, Oriya, Telugu, Sinhala, Kannada, Malayalam, Lao.