I wrote my own preview handler on .NET for Outlook 2007/2010 and XP/Vista/Windows7
When an e-mail with an attachment requires my handler to visualize it an alert says:
"you should only preview files from a trustworthy source"
I tried signing the EXE and the alert is still there.
The answer is setting this key on the registry: HKCR\Software\Microsoft\Office\Previewers\\PONTState = 1