Some SMS providers supply their own not so useful subjects when going through a SMS to email gateway. You can provide your own subject in the message body by including it on the first line surrounded by the hash symbol (#). Note that you must have "Preferred Text Type" set to "plain" to have any chance of this working.
#my subject#
The rest of my message
The subject must be on the very first line and the # character must be the first character in the message (no blank lines, spaces or html). You must also set the "Allow Subject In Mail" setting to "Yes"