The W3S stats for last month (June) show the Linux market share at 4.8%. Sure, it's not 100% accurate but it's nevertheless impressive. The stats only include internet-connected computers (desktops only - servers are not included in these stats) but since there is a fairly big number of computer using Linux which are not connected to the internet, the number may actually be pretty close to the actual Linux market share.
This is a 0.3% increase since last month which even though it might not seem like much, comparing it with the total Linux market share is actually a lot!
W3S stats is the optimistic upper margin. The lower margin seems to still be somewhere around 1%, with marketshare.hitslink.com reporting 1.07% and Statcounter reporting even lower stats: 0.8% - all for June 2010.