I should point out, first of all, that I've searched the manual (pdfs + ctrl-F = awesome), google, some FAQs, and this forum. No joy. I'm probably overlooking something, but dammit, I put in the effort :p

So: at which' levels do clerics gain proficiency points?

A Cleric gains proficiency points like Rogues - once every 4 levels. You can't put more than one point in any weapon or style.

Check out the BG2 section here in Sorcerer's Place, they have some pretty useful docs on game/AD&D rules!

Thankee, sir