Also, any good lawyer you could through this kind of evidence out. It's easy to tamper with (as juise- explained), considering anyone has access to the memory for prolonged periods of time (not really, but the court will only hear this).

However, depending on the laws of the country you reside in, it could be that the mobile phone operator has logs of the texts that were exchanged. Maybe not the content, but time/date, and origin/destination. This is a long shot, and before you get access to the logs, you'll need some good gloves to hold on to 'em bucking horses.

The truth is any phone runs software, on which you can run applications, and which can be modified if you have the right tools (in this case, manufacturer tools). No data is ever secure, unless it's on a secure device (such as a blackberry with high security profile).