I am having this problem too and have been working on it for the last couple of hours to no avail. Have you recently hacked your Mac to use Front Row Enabler from Andrew Escobar? As there is a post
here that blames the missing icons on that. I have tried to follow the instructions on the website to rectify the problem but it didn't work for me.
Also, people on the official Apple forums are reporting this problem
here and
here. I have tried everything it suggests in these posts but it doesn't work for me. I'm glad to know that I'm not the only one going through all of this.
Please let me know if you do come up with a solution.