depends on what you mean... if you mean customizing the interface / the layout of the website, then no, we haven't considered that a viable possibility. for every interface option we provide, we introduce that much more maintenance cost to our already-overburdened team. so every UI update we make has to be considered carefully.
personally i would love to make tumblr a fully customizable interface, but the complexity of doing so is staggering. and very few people would end up using it. unfortunately that means we have to try to find the best common solution for the most people. that's actually why we try to make sure third-party extensions like XKit continue to exist and get our support. i think we're actually missing a wider array of third-party solutions to better fit the specific ways people want to use tumblr.
however, we still care deeply about accessibility and usability concerns, and we're going to take a hard look at the feedback we've gotten about the latest experiment (the new desktop layout) to alleviate as many issues there as we can, that we hadn't already been tracking. the version that's out right now isn't final, and we may find that the overall usability is worse, that's why it's an experiment.
if what you meant was customizing your feed(s) themselves on tumblr, then that is something we're working on a lot right now. in the @labs group we have some explorations into how to customize and curate your dashboard in new and different ways. after all, that's what tumblr is really all about, to me.