I'm having the same problem on a new Windows 8 computer. The dropbox shell extensions worked for me initially, but at some point in the last week they stopped - both icons and context menu items are gone. I've tried reinstalling and rebooting. The shell extensions DO work on a fresh user but that's not a solution for me. I've done a lot of customizing to windows in the last week but can't imagine what I did that would break Dropbox. My other shell extensions (eg Tortoise) still work correctly.
I checked in ShellExeView and see one context and several icon overlay extensions, all look correct:
http://snag.gy/ILuJU.jpg
Any next step I can take to debug this?
Edit: I restored my UAC and ConsentPromptBehaviorAdmin settings to defaults on a hunch. I've gotten it into a state where the context shell extension works right after install, but stops working after I reboot for the first time. I suspect the problem might have something to do with dropbox's integration with the windows 8 store. Which is a hurdle for another day. :/