Well, the separation happened pretty early in the shoe's lifetime (probably less than 3 months in) and I've just been dealing with the annoyance. I just used some Shoe Goo (thanks, Lester) and fixed that part of it. I have to say that my feet have not felt any discomfort, so I'm not feeling compelled to buy new shoes just yet.

Maybe someone can point me to some unbiased studies, but I tend to think the idea that shoes need to be replaced after a certain number of miles is a myth perpetuated by the shoe companies (and repeated by endorsed athletes and coaches) in order to boost sales. That's not to say that shoes don't indeed wear out and need to be replaced, but that depends on a great deal more factors than "miles" put on them. How heavy your footfall is, where you run, the quality of the shoe in the first place, etc. I will replace my shoes for sure, but my feet will tell me when that time comes.