Will the Nintendo 3DS be able to play original DS and DSi games? And if those DS original games have online play (like Metroid Prime Hunters), can you play them online using a 3DS?

I'm pretty sure you can play older games. Don't know about the online thing, though.

yep you can play original ds games and there's some transfer process coming after release to transfer dsi ware games i think. i am not sure about the online thing though 

I'd imagine online will still work since all the online stuff is in the cart itself. Guess it's a wait and see.

Whats funny is i was just thinking about buying a DSi lol. Now im wondering are most new releases going to be for the 3ds? and how long will it be before they phase out the DSi completely. I really dont want to dish out $250 for a 3DS...damn....

The tricky thing about backward compatibility with online features is that they're going to require the same old, outdated wireless encryption that the original DS used. If you're using a newer router with newer encryption, you won't be able to connection to the Nintendo Wi-Fi Connection. This is even the case on the DSi - the system itself supports newer encryption, but the games themselves do not, as that information is in the game card itself.
 
So if I had to guess, the answer to "Will I be able to play Metroid Prime Hunters online still?" will be, "Maybe, if you still use an older router."
 
Games themselves will definitely be backwards compatible.