So I take it there was a change of plan from the statement dhn made on a51
While I understand its your style and you can choose what you feels best suited to it, but thought the move to a free font would welcomed by the community.
Firstly, a phpBB with prosilver bundled hasn't been released, nor has the prosilver SDK. People asked what font was used for the current images... and you got what you asked, Helvetica Neue.
However, the SDK will most probably use Arial (Arial Unicode MS is a good match at the sizes used in prosilver, with a bit of Photoshop font/weight tweaking).
Still, if those doing localised imagesets really want to use Helvetiva Neue (and its Cyrillic & Greek variants, if applicable), they are welcome too. Still, scripts like Arabic, Han (Hanzi (in-Chinese)/Kanji (in-Japanese)/Hanja (in-Korean)/Hán Tự (in-Vietnamese Chữ Nôm)), Hiragana/Katagana, Hangul, Ethiopic, et cetera, will need to have a suitable font found.