I had the opportunity to try out SL on a VAIO SZ-series notebook computer today. (Tech details: Core Duo processor and NVidia 7400 Go graphics. The one I tried had the lowest-end Core Duo -- 1.66 GHz -- the faster versions should be even better.) It worked very well indeed; this is a real breakthrough for a machine in the 4 pound weight class. It's very difficult to find an under 5 pound notebook with decent graphics, but this one looks like it would fill the bill.
The downsides: this is a $2000 system configured for SL. (My preference would be 2GB of RAM, which requires 2x1GB SODIMMs. Sony charges an arm and a leg for that memory configuration; buy it with less RAM and upgrade it yourself. Sony also overcharges for larger hard disks, so consider buying the system with the smallest one and upgrading elsewhere.) It's a Sony, a company that some people are down on these days because of the music CD rootkit fiasco.
I also saw an ASUS system listed on Newegg that looks worth a look. Similar specs, slightly heavier. One minor catch; the first 512MB of RAM seems to be built onto the motherboard, so the RAM ceiling is 1.5GB. Or if you're willing to go a bit larger and heavier (a bit over 5 pounds), you could look at this one. Even that is smaller and lighter than the machines with reasonable graphics and CPU that I had seen previously, which are all over 6 pounds. Sure, a Dell XPS1710 runs SL very well indeed, but do you want to carry it??