If you are worried by the amount of personal data it is possible to store on a mobile, and how easy it still is to lose your phone, then I'd suggest you get some security. There are a number of different solutions to try out including Cryptex which, as the name would have you believe, works by storing your private information in an encrypted, password protected database.
Cryptex is a pretty simple piece of software which works using standard RC4 algorhythm granting access to your phone's private data by means of a security code. This can be anything you choose. And, should you forget it, you can add in a password hint. Don't make this too obvious though, you don't want to be giving the burglars too many clues. There are also nice colourful icons to support which entries you wish to protect. Cryptex means you should now be able to save information like PIN codes and credit card numbers on your phone without worrying. If you're like me though, you'll still take nothing for granted.
Download Cryptex 0.7b in