The shinobi-smart folks over at 9to5mac have posted a way for iPhone and iPod touch users to switch away from the retro 4 number pin passcodes that Apple implements, and into a secure alpha-numeric key:
We've built a profile from Apple's corporate developement kit that allows alphanumeric passcodes. All you have to do is open this link (On your iDevice only!) and you will be prompted to pick a new passcode. You will be required to make a passcode with a mix of letters and numbers and you cannot put numbers in a consecutive order. For example you cannot choose "max1234" you would have to do something such as "max2746." If you ever want to remove this feature simply go to Settings/General/Profiles/9to5mac/ then click remove and confirm. Then change your code back to something numeric.
You can grab the link from their site (above), and they remind you they're not responsible for you putting a hyper-secure passcode in place... then forgetting it. If you do give it a try, let us know how it works for you!