Here's the way I see it. I always have my iPhone when I'm on the go. Being on the go means that I'm probably not going to have time to really listen to any kind of deep catalog music, or entire movies. My music catalog isn't really that big, and 99% of what I actually listen to is on my phone, along with several podcasts and a handful of TV episodes.
Now there are other times where I'm away from my computer, but I have more time to enjoy content of some kind, such as on a flight or in a hotel room. During those times I use my 64GB iPad, which has my entire music collection, more TV shows, and several movies on it. I make certain that any videos on the iPad are different than those on the iPhone, which works to expand my "traveling collection." If I want to watch a video that's on my iPhone, but I also have my iPad with me I use the AirView app (a must have if you have multiple iOS devices). I also leave some space free on the iPad just in case I decide to rent/buy some videos from the iTunes store.
All of this, combined with Netflix streaming, has combined to make me pretty much never wish I could consume some content which wasn't on some device I have with me, even on week long vacations.
I'll see how all of this works out this year (I'm getting the 32GB 4S) in terms of increased storage needed for photos and videos. If I need more space I'll either upgrade to an iPad 3 (I assume 128GB will be coming next year), or upgrade to next years' 64GB iPhone.