Ahem, sorry it's taken me so long to respond...
With a bipole, the rear radiation is in-phase, as in two woofers sharing the same enclosure, one on the front and one on the back, as in Mirage M-1's or Definitive Technology speakers.
With a dipole, the rear radiation is out-of-phase, as in Maggies or electrostats, and usually there is no enclosure.
The out of phase front and rear radiation of a dipole tends to cancel out the bass, so for a dipole to operate as a full-range speaker it needs a large baffle area (the larger the panel the lower the frequency at which cancellation sets in). Successful dipole designers find some way to equalize out a bit of that rolloff, and some use electronic equalization as well (like the Audio Artistry Beethoven). Because of the cancellation and equalization needed to restore flat response into the low bass, a dipole needs to have quite a bit of air-moving capacity to produce the same bass extension and SPL's as a conventional woofer-in-a-box system.
So why the heck bother to go to all the trouble and expense of a dipole? Because they sound so good. In the first place, there are no cabinet resonances or colorations. In the second place, the radiation pattern they produce is conducive to more natural sounding bass than a conventional speaker (which will have omnidirectional bass radiation). A dipole system will have a figure-8 radiation pattern in the bass - and so for a given on-axis sound pressure level, will be putting about 5 dB less bass out into the environment (this figure calculated by Sigfried Linkwitz, designer of the Audio Artistry speakers). So there will be 5 dB less excitation of room standing wave modes to muddy up the bass. If you read the reviews of large, well-executed dipole speakers (Maggie 20's, Audio Artistry Dvoraks or Beethovens, Gradient Revolutions, Quad 989's, Sound Lab M-1's or A-1's), you will find the reviewer always comments on how natural and musical the bass is. Dollar for dollar a conventional woofer system will play louder and deeper, but I have yet to hear a conventional system match the naturalness of a good dipole, probably because the radiation pattern acoustics overwhelmingly favors the dipole.