I've read what I think are the relevant parts of the Aperture User Manual, especially pages 40-42. I'm posting here in hope of getting confirmation that what I'm about to do makes sense and will work.
My main, rather large library of raw files is on my iMac (and is backed up using Time Machine). I'm happy to work on the iMac most of the time but I would like also to be able to do some work on my Macbook Air. I can think of two ways to do this.
Plan A: Dropbox
One method would be to use Dropbox. I have enough storage in my Dropbox account itself (about 125GB, most of it presently free space). And using Dropbox in this way would ensure that my files continue to be backed up, in fact, it would make them extra safe, because they'd get backed up to Time Machine and synced to Dropbox's servers (and my laptop). The main problem with this approach is that I'm kind of in a hurry right now, and syncing, say, a 40GB Aperture library to Dropbox would take a good while — perhaps a day or two. All that data has to be uploaded from my iMac to Dropbox's servers, and then synced from there (i.e. downloaded) to the Dropbox folders on my Macbook Air.
I understand quite well that I wouldn't want to work on the library simultaneously (or even close to simultaneously) from both computers, because it would confuse Dropbox's sync methods. But I wouldn't do that.
Plan B: Portable hard drive
The other approach I'm considering is to export selected projects to an Aperture library (with originals) and put the library on a portable 50GB hard drive. I could plug the drive into my MBA to work on projects there. Then I could dismount it and plug it into the iMac to work there. Eventually I'd use the File > Merge Libraries command to get the remote library back into the iMac's master library.
I don't think I'd be risking too much using this method. As I understand it, exporting a library from Aperture on my iMac (with originals) essentially makes a second copy of everything. So even if the portable hard drive failed and became unrecoverable (unlikely, but if), I might lose any edits I'd done in the library on the portable drive, but at least the original raw files would still be on my iMac (along with any edits I'd made in Aperture prior to the export of the library).
Does this all make sense? I'm leaning toward Plan B (external portable drive) because of its speed.