Yeah, uh, get a laptop that works with linux and has an nvidia gfx card.
Just kidding.
Actually, when it comes to laptop and linux support, it really comes down to the components.
In particular things that might cause trouble are integrated modem, lan, wlan adaptors. In laptops, most integrated modems are winmodems, which range from minimal fuss installation to "are you frickin' kidding me?".
Same could be for wlan and lan. If you can, try and find out exactly which components a certain laptop has before you decide to buy.
Some pages on the net might help you in that respect.
Generally just go to google and search for "linux laptop" and you should find some pages with good references.
Off the top off my head, I think I recall that someone said that ibm thinkpads are good for linux. But I think that in the end it really depends on the particular model, regardless which vendor.