This happened to me as well. In fact it happened twice. The first time, the icons just sort of reset themselves after about 12 hours. No reasoning behind it as far as I could tell. A few days later I powered down the phone, and once again upon restart, the icons showed up only as the white default image. Unfortunately, still two days later, nothing had reverted back and the blank icons remained.
But I did find a quick solution that worked phone. I know this is going to sound wacky, but please give it a try.
I switched the iPhone's language from English to Japanese. After what felt like ages (when in fact it was only just a few minutes), the language change kicked-in and all of my icons had returned to normal. I then reverted my language settings back to English, and the original icons were still there. For now, it seems to be a fix.
My reasoning behind this is that a number a years ago I had a different problem with my 3GS. On that occasion, I was having trouble with my music files naming and ordering. After a lot of playing and iPhone reformatting, by chance I happened upon the same solution. All it took was a change of the ios language to fix those problems.
Please take a moment to try it out. But be careful to take note of which steps you take to change the language. You will need to retrace those steps when you want to revert it back to your native language, and the menus can be a little tricky if you can't read them.