I'm not sure of any laptops that come with Linux as default - ironically it is probably cheaper to get a laptop with Windows XP on and just remove it (companies like Dell/IBM only pay a few quid for it).
If you just want to learn it, why not install it under windows itself? Create a Virtual PC and do it that way
IBM & Dell tried to sell Linux on a Laptop but sales were low ... so I would give them a tinkle and ask.
As for the others ... a word of warning ... you will be hard pressed to find anyone who will sell a laptop without an OS, if you do, let me know. Then there is of course the cost to you to not have Windows bundled with your LT. You will find it costs the same with or without Windows.
Just get a Laptop, with that OS anf dual-boot with Linux ... when your happy bin the Windows bit.
If this topic for you is still active, then I've seen often BenQ laptops with linux as default( i think there are mandrivas instaled) wich will be cheaper than paying for Windows OEM