all of 'em, unless it ridiculously long (more than 10 pages) in which case I read the first page an' the last two 
Background Reading
There are people who reply without reading any of the previous posts.
Oh, someone said that already.
Incidentally, have you noticed how spammers on chess.com are upping their game? They often now open with a random piece of chess-related text, and then include a link to their wares, pish-posh shoes, say.
I'm not sure I remember the chameleon approach here but I have seen it for sure elsewhere. I'm still presuming their trick is to load viruses and malware if you go to their sites.
I hope no one here is waiting for their Nikon cameras to show up in the mail.
edit: I clicked on your link (usually you just underscore for emphasis)--shango7. Man that guy brings me back. Anyway, what?
Oh, someone said that already.

Often I'll read all of them, while sometimes I sift through and read ones that
I think will be funny or insightful.
So who are all these fly-by-night posters that don't read before posting?
I'm wondering who's not so glued to this site that they don't see a forum and have the opportunity to post on the first page. I mean really, people, where's your dedication?
If it's a question (such as the pish-posh link) I answer without reading anything so that my comment will not be tainted by the opinions of others. Then I go back and read what others had to say. Then I post additional comments, explaining to them why they are all wrong! 
Lol, and I will not read your comment because I don't want my opinion to be tainted by anyone else's.

Just out of curiousity, how many earlier posts (on average) do you read BEFORE replying to the latest post in a thread?