As I'm on this I'm on an older IBM T21, does the job well and build quality is good.
Having dealt with all sorts in the last 5 yrs (comp, tosh, dell, sony, etc). I think you can't beat the IBM for quality and "thunkability" (The ability to get whacked and still go). The Dells are good for budget but I feel the build quality is a little less desirable.
Yep wipin' is by far the best way to go. As far as I'm aware it won't void the Hardware warranty, but as you said you may put less than compliant software on it there may be some tricky questions to answer to the support guys
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TIP: Make sure you get a hardware list and have at least enough drivers to get you back on the net after wipin' it.