AJAX is definately one of the most used styles in todays web 2.0 start ups for sure. Actually lot of sites did use DHTML a while ago but the trend is really starting right now... I think the main reason for that would be that if you use AJAX you can sort of give the user more control over the site (and faster speeds) than just having a static page. Infact I was recently trying to AJAX my blog, with prototype.js which is extremly great. And did I mention that since there are a lot of prebuilt frameworks (like Rico and Behaviour and so on) that you can use, AJAX is sort of like piece of cake these days.
AJAX is definately going somewhere, and soon I won't be surprised if all the start ups and so on start using heavy AJAX.