EDIT: ignore my post below, I've just tried again and it would appear that m.facebook.com does now support access via HTTPS.

The problem with the mobile web interface is that it doesn't support HTTPS (unless they have changed that recently). Not a problem when you are on your own wireless or your carriers cellular network, but I wouldn't trust it on a public wireless AP (I've seen people be caught out by session stealing tricks like that illustrated by firesheep - nothing malicious but you know if someone out there is using it to joke about someone else will be using it to cause grief).

Then again, the app may well use HTTP too - I've never installed it to try so I've not profiled it to see what access method is being used.