The free, 30-day trial version of Adobe Premiere Pro CS5 and earlier lacked many of the most useful and popular codecs, including codecs for MPEG-2, AVCHD, and RED media. This meant that people had a hard time evaluating the software for real-world use.
The trial version of Adobe Premiere Pro CS5.5, and later includes all of the codecs that are included with the full version of Adobe Premiere Pro CS5.5. This means that you can import and export to all of the supported file formats using the trial version.
See this page for more details about the trial version.
For lists of formats that you can import into and export from Adobe Premiere Pro, see these pages: