There are a variety of threads on Ubuntu laptops at Ubuntuforums. Here's the main thread:
http://ubuntuforums.org/showthread.php?t=1543006.
This forum focuses on hardware and laptop issues. If you find a model you think you might want to buy, and you can't find any good information about Linux compatibility, I suggest posting a question to this forum.
Most people have good success with machines from Lenovo, Acer and ASUS.
I installed Kubuntu on my daughter's HP dv6t laptop, and it was a pain in the neck if you wanted to preserve dual boot. HP's come with all four primary disk partitions in use so there is no easy way to create a dual-boot arrangement. I documented the steps involved
here.
Things to be aware of include:
1) Wifi adapters - Some have better support than others. I avoid Broadcom hardware if possible and prefer devices that use either the Atheros or Intel chipsets. It's always possible to cobble together a solution using "ndiswrapper" and the appropriate Windows driver, but it's a lot easier to pick a machine with a well-supported adapter. For a good list, see
this site.
2) "Hybrid" graphics - Many new laptops come with two graphics cards, an Intel-based chip that uses less power but has limited features, and an NVIDIA or ATI adapter that eats more battery but has support for 3D and other more advanced features. These machines have posed problems for Linux developers as the specs for these hybrid designs are proprietary, and companies like NVIDIA have chosen not to invest in building good Linux drivers for these machines. Luckily the open-source community has been working hard on this problem. For NVIDIA hybrids, there's the "
Bumblebee" project. For ATI devices, see
this thread.
3) Suspend/resume - Works with some machines but not others. Lenovos apparently suspend correctly. My little ASUS 1201n netbook does not.
4) Fingerprint readers - Generally aren't very well-supported in Linux.
If you have the oppotunity to try out the machine in a store, bring along an Ubuntu CD and see if they will let you boot from it in the "Try Ubuntu" mode.