Mentioning placement offer in a curriculum vitae?
If you are working at the company while you are applying to schools, then it should definitely be mentioned. However, if you are choosing between the placement offer and graduate school (in other words, accepting the offer instead of graduate school), then I would leave it out. This is because this may give the appearance that you're not really serious about applying to graduate schools, which may lessen your odds of getting an interview or an offer.

If you feel it's absolutely essential to mention it, you should do so in your cover letter; however, for the most part, I would do so only if it became important for you to do so (because there's a deadline for accepting the offer, for instance).