If you were planning on being among the first to get your hands on an iPhone 4, we must advise against it, as reports from early purchasers indicate the phone is having hardware issues. Of the reports coming in there are three major issues with the iPhone 4 that standout from the rest.

Glass display shattering or scratching easily:
Unlike past iPhones, which used plastic, the front and back of the iPhone 4 is made of chemically strengthened aluminosilcate glass. This chemically strengthened glass is said to be 30 times harder than plastic and resistant to scratching, however, reports indicate otherwise. It is being reported that the glass on the front and back of the iPhone 4 is shattering, and scratching very easily; even if dropped from no less than a one foot height.
Yellow spots or lines showing up in display:
There is no word yet on what is causing the issue but there are tons of early iPhone 4 purchasers who are reporting yellow spots and lines showing up in their displays. There are currently 45 cases of yellow showing up in the phone’s display and more reports keep coming in.
Loss of reception when touching bezel:
In hindsight putting your antenna on the outside of your phone may have not been the best idea. Another widespread issue that iPhone 4 owners are reporting is a drop in signal when touching the phone’s bezel. It only occurs when you touch the two antenna location on the bezel at once but up to four bars are lost when it happens.
I understand that you may be itching to go out and get an iPhone 4 but if I were you I would wait until Apple responds to these reports.
Gizmodo 1, 2, 3