It happened to me several times to encounter a tedious bug on my iPhone.
Suddenly, with no apparent reason, every non native application crashes immediately after the launch. A reboot in this case doesn’t help. The native applications (mail, safari, settings, etc) simply work fine.
I’ve just realized that an update of ANY existing application or the installation of ANY new application via the Apple store brings the device back into a normal state and all applications behave normally again.
Obviously this is just an ugly workaround, but it seems to work very effectively. See if it helps.