Not sure I can point to where I've read it, but that opinion (don't bother defragging) was voiced long before Win7's release. I've heard similar advice about reg cleaning, which of course hasn't stopped me cleaning my reg, either.
EDIT: It is likely, from what I've read, that regular enemas (deleting temp files, cookies, etc.) do far more for performance than either defragging or reg cleaning. So I do that, too. There should be a file cleanup tool named High Colonic.
I reg clean regularly. I strongly disagree with not doing that! Not from a performance standpoint (although it does do wonders for that), but due to a quirk of MS with the registry. It seems that if the registry gets too big, it will not load - which means windows will not load! I found that out on a client's machine 8 years ago. Microsoft knows about this "quirk"but I have yet to see any KB on it.
As for deleting temporary files, I agree again. However doing so leaves gaps in the hard disk that defragging nicely optimizes which cuts down on file load times, System paging times, and file seek times (seeking all the pieces parts).
Until Bill Gates can prove to me that defragging is harmful, I will continue to do it.