On Android and, I believe, Nokia & WinMo, this should be possible as those platforms have IBM developed applications. iOS probably won't have this feature.

I like this idea as it makes sense for a device that may be shared, like allowing your kids to use your tablet. Many is the time when the kids wipe the Profile or app and I have to walk the real owner through the install steps.

This is an OS function not an app. If someone gets the phone they get all your data. So you would protect only the mail?

I can lock my phone now if I need to.

Locking a phone is different than encrypting the data store. I agree with @2 that you can lock the phone to keep people out. You can also steal the phone, take-out the SD card and get the data. As Traveler can store data on the SD card for easy transport when you get a new phone, using something similar to Lotus Notes local DB encryption would be a great security enhancement. Uncertain though, the impact to battery life for all of the encryption/decryption functions.

Now having stated that, I have not checked in the more recent version to see if this is included already. Just want to be objective to the best of my ability.

P.S. It is probably best to split this into two ideas as they are quite different. You might get more votes for data encryption, rather than just simply locking the app, which is already covered well in the comments. I don't care about the application lock, but I do care about encrypting data. So, I have to vote "No Opinion" as my votes cancel out.