In place preview of files is a great feature of Vista's shell and Outlook 2007. The lack of support for PDF files really got an my nerves. It seems it got Tim Heuer too. Except Tim did something about it. He worked with the folks at Foxit software - who do a lot of tools for PDF manipulation and and produced on. Currently it's Vista only, but it works in the shell as well as in Outlook. The details, download link and screen shots are on Tim's blog
I put "Must have" in quotes, because this is not a Microsoft endorsement. My evaluation so far has been brief and positive; you should do your own evaluation.
Thanks to Arthur of the Unified communications user group for passing this on