UPDATE 2 JULY 2013: Microsoft is informing MVPs that the fix is in, and new help files should be downloadable by (at latest) the morning of 3 July 2013. So get your Update-Help ready to run. More info.
If you haven't recently run Update-Help... don't. There's a problem with the help files that have been produced recently so that instead of:
This affects all parameters - no value types will be shown. This has been reported to Microsoft, and they've acknowledged receipt of that report and are investigating. Personally, I believe the problem may be related to internal-use-only tools that are used to create the syntax section of the help files, so hopefully it'll be an easy fix.
The -full and -detail help still shows the correct information, so if you've downloaded the borked help files, you're not totally out of luck.
As far as I can determine, this only currently affects core PowerShell cmdlets, not add-in modules from product teams like Exchange, etc. I believe that's because the core cmdlets were just updated and re-published, something the PowerShell team tends to do a bit more frequently than some of the other product groups.
I'll keep you posted as I learn anything new.