In most ways no, you cannot get them back. However, if you setup your phone to store all saved messages or if you are on BES, there is a possiblity they were saved.
I don't know neccesarily how to automatically save them but I am sure there is either a way or a program to automate it. It can be done manually as well. If you are on BES then it comes down to having the feature setup.
Hope this helps and sorry about your bad luck.