I use my iPad a lot less since getting my iPhone. It stays on my night stand and gets used daily, but I don't carry it around with me as much as I did before I got my iPhone 4. The retina display makes it possible to read documents on the iPhone 4 (with reading glasses of course).
Edit: I should add that I have NEVER touched my netbook since getting the iPad. While the iPhone is not necessarily an iPad replacement, for me the iPad worked out as a complete netbook replacement. If the rumors are true about a 4 inch display on a future iPhone, if it was retina based we might get iPad resolution on an iPhone sized device. At that point I'd use my iPad even less, but I'd still keep it around as there are times I want the larger screen.