Method 1: (Easy)
If you have some clue about your deleted text, then this method works for you,
Just 1 - 2 - 3 steps.
Interesting? Its true, read on...
Step 1: Backup your iPhone using iTunes
i. Connect your iPhone to PC/Mac
ii. Open iTunes
iii. In the left panel you will see your device name showing under "Devices" category
iv. Right Click and select Backup
Step 2: Go to iTunes backup Location and open the SMS DB file using some Text editor
Default backup locations of iTunes are:
Mac OS: ~/Library/Application Support/MobileSync/Backup/
Windows XP: C:\Documents and Settings\user\Application Data\Apple Computer\MobileSync\Backup
Windows Vista and 7: C:\Users\user\AppData\Roaming\Apple Computer\MobileSync\Backup
Note: In Mac, the MobileSync folder may not be visible by default, so in Finder click the Go menu, now press and hold Option (alt) Key to show the hidden Library menu.
Navigate to Backup folder and open the file named 3d0d7e5fb2ce288813306e4d4636395e047a3d28 using any text editor
Step 3: Search for your message using the clue you have.
Note: This is an SQLite file, so you can also open it using any SQLite Manager to view its tables and data, but you cannot view deleted text.
Method 2: (Very Very DIFFICULT !!!)
If you are running out of time / not able to execute the steps detailed above or too lazy then this method is for you,
The piece of CAKE is ready baked for you here!
I have written a tool called iSMS Recovery that does all these jobs for you.