FSP had a post several days ago (I’m hopelessly behind on my blog reading) about students who don’t let programs know if they will or will not accept an opportunity.
As I mentioned before, I communicated a fair bit with potential advisors when I applied to grad school. I’m not sure how annoying they found this (opinion seems to vary by discipline and professor disposition), but it saved everyone some effort if our early communication determined that it would not be a good fit. So I’d been in touch with professors before and it was a fairly simple exercise to give them a heads up when I made a decision.
My “rejection” note said something along the lines of this:
“Dear Professor X:
Thank you for answering my questions and helping me through the application process [as applicable] at School X. It was a difficult decision to make, but I accepted [gradschool]’s offer of admission and will be working with [advisor] on [some sort of contaminant-related problem]. I will [follow whatever the department requires for a response], but I wanted to let you know personally that I appreciate your time and consideration.
Sincerely, Short Geologist”
Each professor I’d been in touch with sent me a gracious e-mail thanking me for the information and wishing me luck in grad school. I’m not sure how common this is, but I’d recommend sending something personal like this, especially if you’re planning on starting a career in academia and you could see collaborating with folks in the departments you “rejected”.