I am seeing an increasing number of engineers switching to Apple @home while continuing develop under Windows only because their employer uses it.
Also, more and more technical books have screenshots in them done using Mac. While I can't say Apple is "popular", I can certainly say that Mac is getting more popular.
Actually those technical books you mentioned are always around Web Development, Ruby on Rails, HTML/CSS.
Most of Web Designers have background in Graphics Design. Graphics Design use Mac.
Ruby on Rails developers (most of them) use Mac thanks to the hype and the shiny screencast made by DHH.
While the last reason might not be important, it also has some effect to certain degree to books; IE6/WinXP theme looks old (same thing with IE6/WinXP classic theme) while Safari looks much nicer in those web development books. Basically people need fresh air that's all. It's not that Mac is better.
Did you know that Yahoo! Mail doesn't work in Safari?
Also, more and more technical books have screenshots in them done using Mac. While I can't say Apple is "popular", I can certainly say that Mac is getting more popular.