A commenter just pointed me to this post over at John C. Dvorak's blog which gives Firefox nearly 60% of the readership in one stats snapshot. Add in Mozilla and Camino, and Gecko is at 62% -- nearly 2/3rds!! Not bad at all.
After the mainstream browsers, Firefox and IE, alternatives Safari and Opera come in third and fourth with 9% and 5% readership respectively.
I've been tracking Firefox stats around the blogs for a while now and after several surveys of the top 100 blogs, I noticed that the average of the top 100 was tracking perfectly with the stats at boingboing.net, which today puts Firefox at 38.9% (43.6% for all of Gecko,) so now I just use that measure as my measure of Firefox's share of the blog reading market.
The alternative browsers, Safari and Opera, are at 10.7% and 1.9% respectively.