Here's where the strange part starts - in my pre-existing application (which uses Qt 4.6.2, and OpenC/C++), if I just put the following lines into the file:
>CONFIG += mobility
>MOBILITY = messaging
>INCLUDEPATH += C:\QtMobility\src\messaging
>TARGET.CAPABILITY = NetworkServices \
> LocalServices \
> ReadUserData \
> WriteUserData \
> UserEnvironment \
> ReadDeviceData \
> WriteDeviceData

*without any changes to my code whatsoever*, get it signed from symbian online signed, and try to install and run it on my phone, it fails to run, and crashes. The strangest part is, once it has crashed, my whole phone freezes/becomes extremely slow, and I am forced to restart the phone.

Can anyone point out what the possible problem might be, and if there is some way to solve it? I'm completely puzzled.