Here's an interesting revelation - that the white iPhone 4 is fatter (or technically, thicker) than the black iPhone 4.
How much thicker ... about 0.2mm thicker it seems. While that might not seem like much, taking the thickness up from 9.3mm to 9.5mm means that some cases might not fit.
This is interesting because in a matter of a few weeks Apple has fragmented the iPhone case market, first by introducing the Verizon iPhone 4 where the mute switch has been moved, and not a handset that's a little thicker. The difference isn't that much, but it a face is snug for the black iPhone 4, it might be a little too snug for the white iPhone 4.
So if you're buying a case, you might want to check it fits first ... for most cases it shouldn't make a difference, but it's better to be safe than sorry.
[UPDATE: A couple of new white iPhone 4 owners have emailed to tell me that they're having problem with the proximity sensor on the iPhone not working as well (in other words, not blanking the screen when the handset is held up to the face) when screen protectors are fitted.]