Android based tablets does not allow you to send SMS as they do not have the option. But if you use an Android tablet along with an Android phone, you can now send SMS from tablet while connected to phone with Text Tab app. It allows you to connect your Android 2.1+ tablet to your Android 2.1+ phone over Bluetooth and send/receive SMS messages on your tablet as if you were sending them from your phone.

App Features:

• Honeycomb optimized, though not required
• Push menu button to connect devices via Bluetooth, with option to select a device to automatically connect to in settings
• Conversation threads use your Google contact information to display contact info
• Widget with button to enable/disable service, and displays connection information

More details on how to use the app including the download link is available here.