I've done a fair bit of searching on this, but I still have some unanswered questions. I have a 16GB iPhone 4 and we just ordered my wife an 8GB iPhone 4. (We also have an iPad 2 in the household). I'm probably going to start by just using my Apple ID for my wife's phone. Some points to consider:
Apps - Obviously I like not having to buy apps twice. However, I'm sure to have a ton of apps that she doesn't want. So if we delete them from her phone, will iTunes keep them off, or will it push them all back on when she syncs?
iCloud storage - I have 3.6GB free of my 5GB free alotment so I should be able to backup another device and not worry about space.
iTunes - If I went the other way and used two ID's, can you sync 2 iOS devices with different ID's to the same iTunes library? I had never thought about this until now.
iMessage - I've read some pages where folks say that you can't use iMessage on two devices with the same Apple ID. Is this correct? Is there a workaround?
What am I NOT considering? Any there gotcha's or things I need to be aware of?