The following are either my personal opinions or stories from people I trust.
Notice that for me the ability to run Linux and control my hardware and software is essential, which motivates most of what follows. Also note that a large part of it is hearsay, which may be obsolete by the time you read it.
Vendors I like:
I found three basic pieces of advice to be very useful:
As usual, it's a good idea to check your indended Linux distro's hardware compatibility list first, and then look in the newsgroups (Google is your friend).
As time passes, I feel less likely to buy from major vendors. Partly this is due to Microsoft's pressure on OEMs to provide "recovery" CDs tied to a particular brand of BIOS, not a full distribution of Windows. Apart from being tied to a particular machine, these "recovery" CDs likely start their "recovery" with formatting the hard drive to restore the pristine partition table, which is not acceptable for someone who does most of his work in an active Linux partition on the very same disk.
On another count, a Dell machine arrived with a broken video card, and so did not work "out of the box". They were really good about replacing faulty parts, though, in my own experience.
HP will ship them for $10 if your disk has crashed or been upgraded, but you need to call them and then wait until it arrives -- they won't ship them to you just because you want to be prepared, even if you are willing to pay. Not a good prospect if your PC has just crashed, huh? Alternatively, you can ship your computer back to them so that they can re-image the disk (and what happens to your files?)
This means that you cannot easily re-partition you disk either, as you won't be able to rebuild right away if something goes awry. No Linux partition for you, just use Windoze and shut up.
See article at InfoWorld, article on Slashdot -- many interesting customer stories there...
By they time you are reading this, every major OEM vendor may have adopted the same practice. Very sad...