Having a Mini PCI-e slot can be very handy.  When you want to add a WWAN card, SSD Hard Drive, or video decoder using one of these slots will help you do so without adding anything external to your computer.

The data below comes from Pocketables.net.

Dell Mini 9: 3 available in 3G-enabled models; otherwise 2; WIFI is half-height
Dell Mini 10: 3 available in 3G-enabled models; otherwise 2; WIFI is half-height
Gigabyte M912: 2 available
Lenovo S10: 1 available (3G available in Europe; cannot import; 2 slots)
Samsung NC10: 1 available (3G available in Europe; cannot import; 2 slots)
HP Mini 1000; 2 available in 3G-enabled models; otherwise 1
HP 2140: 1 available [2150 model will have 3G]
Acer AOD150: 2 available (3G available in Europe; cannot import; still 2)
Acer 8.9″: 2 available in 3G-enabled models; otherwise 1 (tnkgrl has added a slot successfully)
OCZ Neutrino: 2 available (WIFI is half-height)
Asus 1000HE: 1 available (3G available in Europe; cannot import; 2 slots)
Asus N10: Unclear; appear to be 3, but one seems to be designed for SSD only
Asus 9″ series: not researched
MSI Wind 9″ and 10″: 1 available (3G available in Europe; cannot import; 2 slots)
LG X110: 2 available (3G and WIFI seem to be the only configuration) [AT&T will sell it]
Toshiba NB100: 3G available in Europe; cannot import; 2 slots
Fujitsu U2010/U820: 1 available (free) for 3G, both on WWAN model and original.

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