I bought the Orange Box on Steam as a download about a year ago. I really like being able to log into my account on any computer and being able to re-download the game and play it without keeping track of a disk or DVD key.
Now Portal 2 is coming out. If I purchase it on DVD (I have some Amazon.com gift card I want to use) do I loose any of the cool Steam features that you get with a totally digital game purchase?
If the game is an official Steamworks game there is no difference between the DVD version and the online version, the DVD is just like preloading the game. This means you get all the advantages and disadvantages of Steam, no matter where you buy it.
This is not necessarily the case for games that are just sold via Steam, but don’t use any of the Steamworks features.
As Portal 2 is produced by Valve, the same people responsible for Steam, I’m pretty sure it will be a full Steamworks game and it won’t matter where you buy it. I couldn’t confirm it though, so I can’t be 100% sure.
You can generally register all Valve games on Steam and add them to your library, so this will most likely also be the case with Portal 2.