I've been a Linux user for years - but a week ago I bought a new Dell Inspiron 600m, and it still runs the XP install that it came with out of the box.
Hardware, plain and simple. Particularly wireless hardware. The Intel Pro Wireless 2100 mini-PCI chipset that it shipped with doesn't have a Linux driver available. My Linksys WPC54G 802.11g Cardbus adapter doesn't have a Linux driver available. My only option would be to use my SMC2635W 802.11b cardbus card, which has a binary-only driver from the fine people at admtek, which doesn't have near the range of the internal Mini-PCI or the fast bitrates of the 54G card.
Both the unsupported cards are rumored (the Intel chipset) or promoted (the Broadcom 54G chipset) as having Linux drivers available internally or to select customers, but as yet unreleased to the public. It therefore can't be a matter of time or cost - what's keeping them from releasing binary drivers, at least?
Beyond the scope of this post is Linksys's probable GPL violations in the firmware of the WRT54G.
How do we, as the Linux user community, convince the Big Corporations that there's a demand? I've spoken to reps from Linksys on the phone and they've told me flat out that they don't have any plans to release Linux support for the 54G, now or ever, without citing any kind of backing reasoning.