You can't have photos that you are editing in both locations and keep them in sync with Aperture. The projects will get out of sync, and there's no way to "merge" those changes.
What you *can* do is consider your desktop your "primary" repository, and then use the laptop when you're out on shoots. Take a bunch of photos, import them into Aperture, make your edits (in my case, this is edge sharpening, levels, cloning out way too many dust spots on my dirty sensor, adding copyright and metadata), and do all this work on the laptop. When you get home, export the project with the photos from the shoot. You can then import this onto your desktop machine and all changes are intact. You wouldn't want to sync from this point forward, but it makes it very easy and straightforward to do mobile editing.
Did this just this past weekend:
SmugMug - Bill Lloyd (blloyd) : Eastern Sierra Fall 2006
And it worked great. I also noticed that my 2.16 GHz MacBook Pro seems to be faster for most editing tasks than my Quad G5 with 7800 GT (!).