For this answer, I'll be going by the Rules Compendium's rules on magic items, which are very few and described in Magic Item Level and Rarity. The Player's Handbook has its own rules for magic items which were overcomplicated and entirely replaced. This article on the WotC site discusses the change.
Magic items are not loyal
One can assume that ownership of a magic item is lifted when the owner dies (so a player can use a Sword of the Ancient King), but what about ownership of items when the owner is merely captured/jailed/knocked unconscious? And what happens if a character has an item belonging to a dead owner, who is then suddenly resurrected?
Magic items don't have a sense of loyalty to any owner: just like any mundane weapon, their owner is whoever's wielding them. The exception here may be magic effects that require you to attune to them, but that's not necessarily out of a sense of loyalty.
Of course, you're free to sit down with your players and work out some loyalty rules for magic items if you think it would be fun!
Power usage is limited only by the item itself
If a character has a magic item with a daily power, can a wizard steal it from his hands in mid-fight and use the power against him?
Can a character give the item to an ally and let him or her use the power?
Item powers are pretty straightforward: an item daily/encounter power is just like any other daily/encounter power, and that is it.
The answer to your conundrum is in considering this scenario: Suppose you have an entire trove of magic items with daily powers available. You can go ahead and use up each daily power if you like, and so your character would have effectively limitless access to daily powers. The only limit is your items can only have their daily powers used once each day.
To me, this suggests that if you use your Fireball Scepter's* encounter power to shoot a fireball, the reason that you can't use it again anytime soon is because the scepter has run out of juice. Magic items themselves have a limited capacity for their items to be used by anyone. If you handed that scepter to someone else, it would still be out of juice for a little while.
If someone else takes (or is given) your magic item, they can use its powers. However, they can only do so if those powers have not already been used.
* I just made this item up for the example. It may or may not exist.