Can a capacitor be used as a dipole antenna?
The search-term you want is: "patch antenna."

Yes, parallel-plate capacitors are dipole antennas (especially true at self-resonance, usually up in GHz.)  If the plates are smaller than quarter-wave (or the gap is smaller,) then an impedance matching network would be used to boost the volts for electrically-small dipole-mode.

But self-shielding capacitors such as wrapped cylinders won't make good antennas.  They're not symmetrical, and one plate isn't exposed to the outside world.  They're still dipole antennas, but mostly because of their connecting leads, and they're shorted out by relatively gigantic capacitance.

A very recent innovation in iphone antennas from Fractus Inc. is a tiny 2mm cube with metal faces, plus a series inductor for resonance.  It's intended for upwards of 5GHz, see "ground-plane booster:"

http://www.microwavejournal.com/articles/29138-antenna-less-wireless-a-marriage-between-antenna-and-microwave-engineering?v=preview

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=5723706 (PDF)