I play pretty much exclusively on my laptop, a Dell Inspiron 1520. It has a 2.2 GHz Intel dual-core processor (T7500) and GeForce 8600M GT graphics. Although both could be faster, they do a pretty good job taking care of Civ3 and especially Civ4 (which doesn't scale up as well for huge maps, and thus uses less CPU power at the maximum). If they don't suffice for Civ5, I'll wait to get Civ5 instead of spending for a new computer. But I'd be surprised if it won't play Civ5 well.
On new laptops, a Mobility Radeon 5450 would probably be close to equivalent for the GPU (and a 9500 GS is exactly equivalent from nVIDIA, I have no idea about their newer cards, though), and either a similarly or slightly lower-clocked Core i3, a similarly clocked Core 2, or a slightly higher-clocked Turion for the CPU.
The only thing I miss from playing it on a desktop is the NumPad, which is really nice for quick moves on the isometric or square grids of Civ3 and Civ4. But that won't matter with Civ5, unless someone comes out with a hexagonal num-pad-ish accessory for Civ5.