Definately IBM Thinkpads are not going to be the optimal laptop for Linux.
Laptops are funny.. Almost none of the laptops 'OEM's like Dell, IBM, Gateway and probably even Alienware make their own laptops. They buy their laptops 'bare bones' style from 'ODM's usually located in southeast Asia. They add the CPU, the harddrive, and then the OS.
So many of the times you can find generic or smaller brand that has the same exact parts as the big guys, since they all buy it from the same place. Sometimes they have cosmetic differences to disguise the similarities and such, but internally they are the same.
There are places that do sell Linux laptops/notebooks. Places that sell 'namebrand' notebooks with Linux installed on them are a bit of a rip-off becaue then they'd just buy it from Dell, remove windows, then install Linux on it.
Now linux has ok laptop support nowadays, especially now since the Intel supports Linux for 'centrino' style notebooks.
So that leaves you with a few choices.
Intel-based i855 video cards are supported OK. Each manufacturer has their own Bios they put on things sometimes that can cause issues. So go to 
http://www.linux-laptop.net/ and check out peoples reviews.
they are the standard shared-memory designs for adiquate desktop usage. Very limited for games or 3d performance, require some tweaking to get good performance from playing DVDs and such.
They have the advantage of being supported by open source drivers.
Intel is now, I beleive, starting to sell i915-based notebooks more and more now. These are new, however Intel released the specs to the people at the DRI project a LONG time ago and it should have good 3d support right now. But since they are new  would expect some issues and some power management issues until people have had more time on them to get rid of the bugs. However they are suppose to have much better performance then the i855 stuff, although I don't know for certain.
You'd find these most commonly in low end small laptops or the very thin and light catagories due to their small form factor and low energy needs...
like HP's Linux laptop
For video cards if you need some 3d performance you have open source support for ATI cards up to the 9200 models, and it should work out of the box.
A good example is Dell's 600m models, which are suppose to be good linux laptops.
see here
It has a ATI 9000 and a good display. While slow by modern gaming standards it's quite a nice improvement over i855 cards and has dedicated 32megs video card memory. (check out Dell's refurb website. you can find them at a low price.)
If you need more power and don't need much in the way of mobility, then the desktop replacement with AMD64 proccessor, Via chipset, and Nvidia video card is the way to go. They are small enough to carry around on trips and such, they have good enough batteries for limited times away from powerplugs, but they aren't something your going to want to lug around from class to class.
Be sure to check out 
http://www.linux-laptop.net/ for reviews on different laptops.
Also check out places like 
http://www.powernotebooks.com/ that actually do sell Linux laptops. Some are very high quality. Some are crap.
http://tuxmobil.org/reseller.html
http://tuxmobil.org/laptop_oem.html
Personally I lbought a Apple Ibook.. because I wanted a PowerPC, no linux company had any small form factor at anywere nearly a acceptable/affordable price, because in a choice between buying a book with Windows XP vs OS X I'd rather choose OS X.
But there are sucky things about. No java plugins, no flash plugins. Closed source software won't work 90% of the time. No PCMCIA slot, no wifi in Linux. Good 3d performance though and good battery life, small well built and actually pretty cheap for what it is. 
Otherwise I would of gotten a Dell 600m, I think. I wanted something fairly small. 
edit:
With a Dell 600m you should have the option of either using the free software DRI drivers or the ATI closed source stuff.  Also pay attention to what wifi card you choose, the Intel centrino is desirable (from what I've heard, but I haven't used it) over the others because it's support now with open source drivers (which previously it wasn't for a long time.). If you end up with a broadcom unit then the only choice is to use Windows drivers thru the ndiswrapper.
In general for most laptops  for the touchpad 
you'd want to install those drivers to get some of the more advanced features. Last time I used it there was no need to patch the kernel to use them in 2.6. They are for the Synaptics touchpad, which is what most manufacturers use in their laptops. (not to be confused with the synaptics front end for Apt-get)