My run of the mill thinkpad has operated at the summit of Mauna Kea (~14,000 ft) many times. I've personally also used a Gateway laptop, and a couple of seagate 2.5" harddrives up there. I know of many people who have used other laptop models there as well, and have never heard of any problems. The pressure difference between 14000 ft and 150000 ft is only about 15%.
I strongly suspect Dell just pulled the 15,000 ft number out of their rear because some marketing person thought it sounded neato.
"Indecision is the basis of flexibility" -- button at a Science Fiction convention.