How do quarter wave antennas work?
The last cell phones I opened up had a loaded bent dipole antennas or J-pole looking antennas.  Of course there are tons of antenna styles possible.  Cell frequencies are from 900-1900 MHz, so 1/4 wave is 1.55-3.3".  Without loading, the antenna would be longer than the phone.

Looking at the document Kortuk linked, it looks like they would be Inverted-F.  Not sure how those vary from J-poles, as they look almost exactly the same.  It might be that dual frequency phones can have Inverted-Fs that work as dipoles and the lower frequency.  RF is sort of like a voodoo art.  :)

If the antenna is balanced, you should not get reflected power so the noise induced in the ground will be low, just the power required for the signal.  You can't bypass the antenna with caps, or you will "short out" the antenna at RF frequencies.  This is no different than noise that digital chip would induce on ground by driving a fairly large load with fast digital pulses.  The filtering needs to be done where it matters, at the power pins of the sensitive parts of the circuit.