I mentioned in a comment on the druid post that I'd have to do a post about weapon restrictions. It's also a particularly important question for thieves, since the standard old-school versions of the thief list a bunch of weapons that are hard to characterize. I've been toying with several different ideas on how to handle class restrictions for months, if not years. I've finally reached these conclusions:
Magic weapons are limited as described in the LBBs. A magic sword is not magical when picked up by a cleric. No magical weapon is magical for a magic-user, except for a magic dagger or magic staff.
Only blunt one-hand magical melée weapons are magical when wielded by a cleric. Only magic short swords, magic crossbows, and small magic melée weapons are magical when wielded by a thief.
Zero-level NPCs only get their HD bonus on attacks using weapons they are trained to use: natural attacks (punch, kick, bite) and maybe two weapons, rarely more than two. They can be trained by an employer to use additional weapons.
Clerics, Magic-Users, Thieves, and most other classes are the same: they only get their HD bonus on weapons they've been trained with. Lawful Clerics also have religious vows they must abide by, but in theory could train with other weapons. To make things simple, first level characters are trained in whatever weapons they paid for during character creation; other weapons require training. If they use any other (untrained) weapon, they attack as a zero-level NPC.
Fighters do not need training; they can use any weapon they pick up.