This is work in progress since a mobile phone with terahertz capability does not exist yet but could potentially be very useful.
Terahertz waves are those in the electromagnetic spectrum that are between millimeter waves and infrared, with frequencies between 0.3 to 3 Terahertz. Currently the technology to generate and analyze them do not exist very well but Dr. Kenneth O, from University of Dallas could change that by using CMOS chips for generation and analysis of Terahertz radiation. T-rays have some very interesting properties:
They have low photon energy and do not damage tissues and DNA and thus can be used to penetrate tissue and analyze for density of the tissue – This may be very relevant for breast cancer detection since they can pass through fatty tissue very easily. They have also been advertised to see through cloth and walls which enables its use in security applications.
This is a very active area of research and in Dec 2012 Caltech have also developed chips and integrated planar arrays that can enable Terahertz use in many devices. However, lots more needs to be done to separate the hype of terahertz from its practical applications though its ability to show effect on tissue culture cells and methods for detecting carcinoma are very interesting.