A few days ago Garmin posted updated firmware (version 4.0) for the StreetPilot c550. I've downloaded and installed the update, and I encountered no issues.

Changes made from version 3.30 to 4.00:

• Added SMS / Text Message Support for compatible phones
• Added support for reading messages with TTS voices
• Added support for importing M3U playlists. Place M3U playlist in the same directory as your music. From the MP3 player, touch Browse -> Import Playlist. Touch the playlist you want to import, and the songs are loaded in that order.
• Changed the safety camera alarm rate
• Enable filtering of the Daylight Saving list based on time zone
• Fixed Switch to button on active call page to be active
• Modified to power off the unit when disconnected by computer while exiting from mass storage mode and restart the unit if the USB cable is pulled
• Added support for Russian, Turkish, Slovak, and Estonian pre-recorded voices
• Added Arizona time zone

Garmin recommends installing the update via their WebUpdater software. If you like the old-school method better, you can grab the file manually HERE.