As previously mentioned according to Matt Janssen, a bug was discovered on the iPhone 3.0 Spotlight, which meant that the search feature kept the emails in the index.

Janssen, claimed they were kept even after being deleted from the inbox and purged from the trash, in the first video Janssen claimed that he found emails that had been deleted months previously, using this method.

The email is more than likely cached to enable speedier searches, tapping on a deleted email would result in the launch of Mobile Mail which would subsequently crash, but to tap a second time would actually show the email albeit in a buggy fashion. It seems now that the iPhone 3.0 Spotlight showing emails that have been deleted has been crushed in iPhone 3.1.
Source – theiphoneblog.com