I also think I may need to allocate a little extra time for talking to my colleagues, letting them know what's up, reassuring them that I'm fine -- I had some nice conversations today, which I'm actually quite glad of, but they do take time. It would be more efficient to e-mail the whole department: "Hey, you may not have heard that I was diagnosed with breast cancer in February. I've been undergoing treatment since, and while it was a rough summer, I'm doing very well now, and the prognosis is very positive. I should be done with treatment by the end of November." But probably what will actually happen is that I'll just show up to the start of semester department party and talk to some people and the news will percolate outwards, because that's how this sort of thing seems to work. :-)
8/11 update: Amended to note that at the recommendation of several academic friends, I ended up sending a note to the department after all. Hopefully, it'll help ease the social awkwardness of such things. :-)