[pjc50]

JTAG is usually an extremely effective way of breaking into the phone, but it's usually used as a first step in reverse engineering rather than for the end-user.

Of course, some phones (Apple) are glued shut which prevents end-user modding.

[rsynnott]

iPhones aren't glued shut (though the battery is glued down in the newest one): http://www.ifixit.com/Teardown/iPhone+5s+Teardown/17383

You may be thinking about the HTC One, which is almost impossible to disassemble without destroying it (though this has improved a bit in the M8).