Here's the General Rule: If the bonuses are untyped they stack. If they are typed, then they don't stack. Usually an item will say "power bonus" or "feat bonus" or "racial bonus", or "item bonus". And you can only have one of each. Some items don't say. These are called "untyped".
If one item gives you a +2 power bonus and you have another item giving you a +3 power bonus, than you only get to keep the +3. Because both are power bonuses.
If you have an item that gives you a +3 Power Bonus and a feat that grants a +2 feat bonus, those combine to give you a +5.
If the bonus is Untyped (ie, it simply says "you get +2 damage when wielding this item" it stacks on to everything. ** modified in PHB3 pg 223 ** [ Howewer, if you gain multiple untyped bonuses from the same named game element (a power, a feat, a class feature, and the like), only the highest bonus applies. unless stated otherwise.]
Specific beats general. if an item/power/feat specifically says that it is a typed bonus that can stack with another typed bonus.. then that is what happens.
For the actual Rules references on this go to PHB1 page 275 "Bonuses and Penalties" and untyped bonus in the glossary of PHB3 pg223.