Almost all up until quite recently, and even now a significant proportion of the damn things.
Admittedly a lot have PowerVR graphics and are a nuisance to use Linux on anyway, but there are still a lot of potential and actual users there.
Also, there are still a fair few Pentium M laptops (and some P4 desktops, although I haven't seen many recently) in existence - mostly with XP*, and not really suited to Vista or Win7 but quite happy with some XFCE/LXDE distro. Quite a lot of those are owned by students etc, who are much more likely to be using Linux than your average PC owner*.
*NB: Such statements are based on what I see as a permanent resident of a university town in the UK, and might not properly represent global trends. I don't see why they shouldn't, though.