Here's the substance of what I said, in the hope that it will be useful to others:
I'm a two-machine guy now. I have been for a while, but I've always looked on each machine as being equal, but for different purposes. With the Air, I'm moving more towards the Mac Pro being home base, and the Air being something I take with me but don't depend on. In that sense it's more like the iPhone: a portable copy of the data, with everything synced up back home.
My photo libraries haven't fitted on a laptop drive for some time, so that hasn't been a particular issue. All I use my laptop for is downloading photos into Aperture whilst travelling, which then get moved back to the Mac Pro when I get home. I don't keep photos on the laptop at all: if I want to show pictures, I'll pull up a Flickr slideshow.
I have always managed my iTunes on the laptop, though, and my home base/portable version model is slightly different here. I look at iTunes as a mixture of static and dynamic media. All the stuff you paid for (music, movies, TV) is static content that you, in principle, want to keep forever. Podcasts are dynamic content which likely get deleted after one or two listens and which you can always get back for free.
I use my iPhone primarily for video and podcasts and don't keep music on it at all. I have all my static media on the Mac Pro and the Air's iTunes library is purely for downloading Podcasts and syncing to the iPhone. I did this because I figured that I wouldn't want to be away for a couple of days and not be able to move new podcasts to the iPhone. I can wait for the other stuff, although I could equally move a video onto the iPhone from the MacBook Air if I needed to whilst away from home.
To get video onto the iPhone, I connect it to the Mac Pro. The iPhone is set to "manually manage music and videos", and I can then just drag over what I want. It is slightly annoying to give up the auto-syncing of the 'newest unwatched' TV shows, but it's not a major pain.
If I do want to take my entire library with me, I use my old 80GB 5G iPod. The beauty of this is that you can play music and movies off an iPod on the MacBook Air screen, although I think I have noticed that doing so does not update the "New/Not New" status of TV shows in the iPhone's internal database. If I ever need to get an audio file off the iPod, well, PodWorks is never far away.